Efficient Handling of Large Storage: A Comparative Study of Some Disk File Systems

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1 Computing For Nation Development, March 10 11, 2011 Bharati Vidyapeeth s Institute of Computer Applications and Management, New Delhi Wasim Ahmad Bhat 1 and S. M. K. Quadri 2 1,2 P. G. Department of Computer Sciences, University of Kashmir 1 wasim.ahmed.bhat@gmail.com and 2 quadrismk@hotmail.com ABSTRACT Since the invention of magnetic disk, many changes have been made to its hardware technology to increase its performance and capacity. Although disk performance has improved but is relatively less as compared to continuous increase in disk capacity. As such, the design and development of new disk file system and refinement of old ones will need to focus on efficient store and retrieval of data for large storage disks. In this paper, we try to compare some common disk file systems on the basis of Storage Limits, Metadata, Characteristics & Allocation Strategy to point out the file system that can serve as a template for future file systems and old ones to support large storage capacities. KEYWORDS, Storage, Comparison 1. INTRODUCTION system is an important part of an operating system as it provides a way by which data can be stored, organized, navigated, accessed and retrieved in form of files and directories from storage sub system [1]. It is generally a kernel module which consists of algorithms to maintain the logical data structures residing on the storage subsystems. The basic key functions that every file system incorporates are basic file operations like copy, move, create, delete and rename, efficient organization of data for quick storage and retrieval and efficient use of disk space. Apart from these basic functions some file systems also provide additional functions such as compression, encryption, file streams and others Magnetic disk drive is the most primitive and cost effective storage device. There has been continuous improvement in its hardware technology to increase its performance and capacity [2]. Although performance has seen less improvement with respect to capacity, but the tremendous drop in cost per unit byte, reliability over solid state storage and increase in capacity have made disk drives every body s choice [3]. And hence, disk file systems have attracted researchers over the globe to exploit its pros and minimize its cons. Our goal is to study and compare the features of modern disk file systems to identify the file system whose design can support large storage disks and thus can provide a template design to support large storage disks. The comparative study is carried out in three phases. Each of the phases consists of activities to be completed within that phase as follows: 1. Initial Phase: This phase is the starting phase of the comparative study. It consists of two activities. Identification: A huge list of file systems exists and a few popular disk file systems have been identified that are predominantly in use. Grouping: The file systems are grouped based on their manufacturer and operating system support. 2. Analysis Phase: This phase is the intermediate phase of the comparative study. It consists of the following activities: Gathering Information: Once the disk file systems have been identified, information about the file systems are gathered and recorded. Advantages and Limitations: Once enough information regarding the identified file systems have been recorded, the advantages and limitations of each file system is analyzed. 3. Comparison Phase: This phase is the final phase of the comparative study. It consists of the following activities: Intra Group Comparison: A comparison is done on the basis of features within the groups already identified in Analysis phase. Inter Group Comparison: A cross group comparison is performed based on features of the groups of disk file systems considered as a whole. For our study we have identified a few modern file systems manufactured by different organizations with different operating systems as their base. The selection is based on the popularity of the file system and their corresponding operating system. The file systems identified for the study are: 1. Second Extended (ext2) 2. Third Extended (ext3) 3. Fourth Extended (ext4) 4. Hierarchical Standard (HFS) 5. Hierarchical Extended (HFS+) 6. Allocation Table 32-bit (FAT32) 7. Extended Allocation Table (exfat) 8. New Technology (NTFS) 9. VERITAS (VxFS) 10. Zettabyte (ZFS) All mentioned disk file systems play an important role in the world of data storage. The identified disk file systems can be grouped based on their manufacturer and the base operating system. The groups are as follows: Linux s: The file systems in this group are basically designed for use with Linux operating systems. The file systems under this category are: 1. Second Extended (ext2) 2. Third Extended (ext3)

2 3. Fourth Extended (ext4) Macintosh s The file systems in this group are proprietary file systems developed by Apple Inc. and are desired to run along with Macintosh systems. The file systems under this category are: 1. Hierarchical Standard (HFS) 2. Hierarchical Extended (HFS+) Windows s The file systems in this group are developed by Microsoft Corporation and runs along with Windows as well as a few other operating systems as well. The file systems under this category are: 1. Allocation Table 32-bit (FAT32) 2. Extended Allocation Table (exfat) 3. New Technology (NTFS) Solaris s The file systems in this group are developed by VERITAS Software and Sun Microsystems and are in use with the Solaris operating system. The file systems under this category are: 1. VERITAS (VxFS) 2. Zettabyte (ZFS) 2. ANALYSIS PHASE In this phase we make a quick review of the basic design of identified file systems so that the pros and cons are highlighted. We begin with Linux based file systems, followed by Macintosh file systems, Windows file systems and finally Solaris file systems. Second Extended (ext2) Second Extended file system [4] was introduced in Linux to overcome the incapability of Extended in I-node [5] modification and timestamps management of data variation. Ext2 is simple, robust, and fast using less number of CPU cycles. The files, directories and objects in the file system are marked by I-nodes which account to be fundamental structure of ext2 file system. As the file system is divided into blocks the number of disk seeks in case of huge amount of data is reduced which results in making ext2 considerably fast. Ext2 is recommended for use in the solid state drives due to its lesser number of disks seeks and writes than other file systems. The major disadvantage of using ext2 is its inability to unmount properly in case of an improper shutdown, power failure or kernel crash. The file system cannot be used till it gets repaired with the utility fsck [6]. If the file system is large, the time required to repair it is also more which results in the system to be unavailable for that period of time. During repair there are also chances of data loss. Third Extended (ext3) Ext3 (Third extended file system) was introduced as to remove the major disadvantage of ext2 i.e. the problem of data loss and file system check after a system crash. Ext3 [7] is a journaling file system which assures system availability and data integrity much more than that of ext2. Journaling file system are those which logs all the changes made to the file system during disk write and uses it to reconstruct or repair the file systems on the fly after situations like the kernel crash or power failure. The time required to repair the file systems depends on the size of the journal rather than that of the file. One of the other most important advantages of the ext3 file system is that it can be easily rolled back to ext2 file system and vice versa. Disadvantages of ext3 file system are: 1. ext3 file system doesn t have a defragmentation tool, 2. ext3 fills in zero with respect to the pointer positions in I- nodes of the deleted files which makes the recovery of those files impossible, 3. ext3 file system doesn t provide transparent compression, 4. Any accidental modification or corruption of the stored data is not detected by ext3 file system as it doesn t calculate checksum while committing into a journal. This results in file system corruption due to a sudden crash. Fourth Extended (ext4) Ext4 is a journaling file system which came as a successor to the ext3 file system and is known as Fourth Extended system [8]. As ext3 provides indirect block mapping in which there is one to one block mapping it serves as inefficient for handling larger chunks of data. Ext4 introduces the use of extents, which is a range of contiguous physical blocks represented by a single descriptor thus reducing the fragmentation and large file performance issues. Ext4 is backward compatible and thus both ext2 and ext3 can be mounted as ext4. Ext3 writes one block at a time which is not efficient in case of larger data handling. For improving the performance and reduction in fragmentation and CPU cycles, the ext4 file system uses delayed allocation technique which allocates blocks on page flush time rather than write time thus clubbing many block allocation request as a single one. Media streaming and database related information needs contiguous memory location and thus ext4 allows persistent pre allocation of the memory which helps in allocating contiguous memory for files irrespective of the order in which the file gets written. Due to more and more highly critical application getting build over Linux it s not accurate to use the second timestamps and thus for it ext4 measures timestamps in nanoseconds. Ext4 also removed the other drawbacks present in ext3 file system as listed below subdirectories can be created in the ext4 file system irrespective of in ext3. 2. Ext4 uses checksum in the journal which improves the data reliability and integrity. 3. e4defrag is a defragmenter tool for ext4. One of the major disadvantages of the ext4 file system till date is due to delayed allocation mechanism in which there is a potential risk of losing the data if the system crashes before page flush time. Hierarchical Standard (HFS) Hierarchical was developed by Apple Inc. primarily to replace their older file system MFS (Macintosh ) because of performance issues in it [9]. In HFS, the volume is divided into logical blocks of 512 bytes which are grouped together into allocation blocks containing one or

3 more sectors depending upon size of volume. There are five components which make up the HFS volume [10] in which logical blocks 0 and 1 are those which are known as the Boot Blocks as they contain the system startup instructions. Logical block 2 is the Master Directory Block which stores the metadata about the volume. Logical block 3 is the starting block Volume Bitmap which keeps track of allocation of allocation blocks by representing each allocation block by a bit. The most important structure of HFS is a Catalog that contains record of all the files and directories stored in the volume. In MFS, this information was stored in a flat file which is replaced in HFS by a B-tree to make search fast. One of the major disadvantages of HFS is that all of the file and directory records are stored in the Catalog file data structure, thus during multitasking it will lead to issues which will lower the performance grade of the file system as all programs will get into the queue if single program writes into this structure. By any means if the Catalog file structure gets destroyed the whole file system comes down. HFS doesn t provide reliability and involves high risk and performance issues taking into account the high volume of data storage capabilities involved with present day files. HFS uses a 16 bit value to address allocation blocks, limiting the n umber of allocation blocks to Hierarchical Extended (HFS+) HFS Plus uses 32 bit block addresses with respect to 16 bit block address of HFS which helps in supporting the larger number of files and larger sized files. HFS Plus introduces journaling which leads it to become much more reliable regarding data integrity and availability. The filename encoding was also changed from Mac Roman to Unicode to support mixed script name and International friendly file names [9]. HFS Plus volumes are split up into nine components [10] among which five are similar to HFS. The Logical blocks in HFS are known as sectors in HFS Plus. Thus the Sector 0 and 1 stand as Boot Block, Sector 2 as Volume Header which is similar to Master Directory Block in HFS. Sector 3 has the Allocation which is similar to Volume Bitmap in HFS. The Extent Overflow and Catalog file remain the same as of HFS. HFS Plus adds the startup file which is useful for the system s which has no HFS or HFS Plus Support. Attribute file is the new inclusion in HFS Plus which can store three different types of records such as Inline Data Attribute record; Fork Data attribute record and Extension attribute record. The last but one section stores the alternate volume header and the last sector is reserved by Apple for its own use. Allocation Table 32-bit (FAT32) FAT32 was the extension of Allocation Table file system which was developed to overcome certain limitations in the FAT16 file systems [11]. The volume size of FAT16 file system was one of the reasons of introduction of FAT32 which had its cluster values embedded in a 32 bit field instead of 16 bit field. It s a very simple file system which can get along with most of the OS like Linux and Mac. The first sector of the drive formatted with FAT32 file system will contain DBR (DOS Boot Record) which contains the boot code and volume information. It is followed by some unused space called Reserved Sectors [12]. Two copies of FAT ( Allocation table) follow the reserved sectors which is a linked list of clusters pertaining to some file. Rest of the file system is filled up by clusters to hold all the files and directories. Advantages of FAT32 [13] file system are as follows: 1. Small clusters are used by FAT32 (4 KB for drives up to 8 GB size) rather than that of FAT16 resulting in saving valuable disk spaces. 2. Drives formatted with FAT32 are less failure prone and are quite robust than FAT16 as it can relocate the root folder and use the backup copy of the file allocation table in case the default one is corrupt or un available. There are few disadvantages of using FAT32 file system which are listed below: 1. The cluster size cannot be increased in FAT32 for making it use clusters less than 65, There is no inbuilt compression and system security in FAT32 file systems. 3. The maximum supported file size is 4 GB in a FAT32 file system which is very less compared to the present day growth of data. Extended Allocation Table (exfat) The Extended file system (also known as FAT64) came as a replacement to the existing FAT32 file system. All the drawbacks of FAT32 file system were taken care of in this version. It is intended to be used in Flash drives and mobile portable storage devices due to its ability to handle large files and large volumes, and interoperability between portable devices and desktop systems. exfat supports files of sizes up to 64 ZB. Free bitmap method is used by exfat for faster free space allocation i.e. every single bit corresponds to a sector and thus checking every bit helps to check the sector is free or not. It also helps in reducing fragmentation. In exfat file system the support was increased for the number of files per directory, than FAT32 file system. Features like journaling and Security access control lists are unavailable in exfat file system. New Technology (NTFS) New Technology system is a journaling file system for Windows OS which is reliable, fast, highly performing than all other traditional FAT file systems [14]. NTFS system contains a MFT (Master Table) in which there in an entry for every file, their attributes, timestamps etc. The MFT size varies with the size of the file system. One particular MFT gets freed when related files get deleted from the system. NTFS file system has quite a large number of features [15] which are described below: 1. NTFS file system can log all the changes in the files and directories and various related attributes to it. This increases the system reliability and recoverability as after a sudden system crash this record helps in to restore back the data in a fast and efficient way. 2. NTFS uses LZ77 (Lempel-Ziv 1977) Algorithm [16] for file compression. Although read/write access is allowed for compressed files but system files are recommended not to be compressed as it puts extra burden on the processor.

4 3. Additional Data Stream allows adjoining extra information with a file without it getting included within the file size. Sometimes this type of information is used by many products. Sparse files are datasets which are mostly dumped with zeroes. These are sometimes used by the Database applications. 4. Volume Shadow Copy helps in taking the backup of the file system and getting back an older version of file quite easily as it stores the historical version of files and folders in it. 5. Encrypting s (EFS) provides strong encryption to the NTFS Disk partition. It uses a symmetric key which is used to encrypt the data. The key is then encrypted with a public key and stored in Additional Data Stream of the file. Decryption uses private key of the user to decrypt the symmetric key, then that is used for decrypting the file. It provides a great lot of security to the file system. 6. Setting of Disk Quotas helps the admin to manage user space limit and usage. 7. Hard links helps you to have a single file in multiple locations without duplicating the files which promises effective utilization of space in NTFS. One of advantages of NTFS is that it can support large volumes of data which helps to manage the on growing data efficiently. VERITAS (VxFS) The VERITAS (VxFS) is an extent based file system in which contiguous blocks known as Extents are reserved for the file [17]. Whole extent gets allocated when a file is written and blocked for the use of that file. When the contents of the file get added afterwards, it starts from the same point in that extent where it left off earlier. This helps in minimizing fragmentation and scattering of files. VxFS was originally developed by VERITAS Software and was used as primary file system on HP-UX OS. VxFS was the first commercial journaling file system. VxFS results in faster I/O operations with the help of aggressive I/O clustering policies and thus allowing system administrator to assign specific application parameters on the basis of per file system. Application can control extent allocation with the help of the extent attributes. VxFS includes the feature of Intent Logging in which the pending changes of the file system are recorded. The changes are recorded in the Intent Log and thus after any sudden system crash the fsck utility scans for the Intent Log rather than the whole structure of the file system, taking very less time for completing pending actions and getting recovered. As VxFs is a journaling file system, it synchronously updates the changes in data and Inodes related to files, providing data integrity. VxFS provides the feature of minimal downtime incase of online backup and administration by avoiding un-mounting of the file system during the process. VxFS also provides defragmentation tool for removing the issues of fragmentation, if any. Zetta-byte (ZFS) Zetta-byte (ZFS) designed by Sun Microsystems is a combination of both file system and logical volume manager [18]. It was designed to support high storage capacities, integration of file system concepts and volume management, automatic repair, etc. and capability to be ported between two different platforms. It is named as Zetta Byte as it can store 2 58 zettabytes, where each ZB is 2 70 bytes. Features of ZFS are listed below: 1. ZFS file systems are built on virtual storage pools called ZPool which is totally different than the traditional file system which uses single device and volume manager to manage the other devices. ZPool contains a series of virtual devices which contains block devices like files and hard drive partitions. ZFS decouples the physical storage from the namespace leading to multiple file systems sharing a same pool of storage. As ZFS integrates the functionality of the volume manager in the file system level operation, it takes care with respect to file systems and file rather than blocks or devices. The pool storage can grow and shrink automatically in ZFS. 2. It is a 128 bit file system which provides immense capacity in terms of data (256 quadrillion ZB).There is absolutely no limit to the number of files and directories. It allows concurrent file read, write operations without violations. 3. It allows all devices in ZPool to share the workload and hence balances the write load across them. 4. ZFS follows Copy on Write Transactional Object model in which all block pointers contain 32 bit checksum of target block. A new block gets allocated and written than overwriting the active data block requiring modification and then any metadata block referencing it is read, reallocated and written. The prime advantage of this model is writing in new data doesn t hamper the blocks containing old data, thus data can always be retained. 5. Copy-On Write Model allows a snapshot of the file system. The data composing the snapshot is already stored and thus snapshot takes very less space and gets created quickly. 6. ZFS file systems can be created much faster than compared to any other file systems. A typical comparison between file creation time of ZFS and NTFS yields that ZFS file system creation takes 17.5 seconds with respect to 4.5 hrs in NTFS [19]. 7. ZFS provides built in compressions techniques for increasing the file system performance. ZFS also uses variable-sized blocks of up to 128KB 8. Acquires the benefits of journaling file system like reliability and security and speed. 3. COMPARISON PHASE The comparison phase consists of two activities. Firstly, the file systems are compared to their group members and we select the most efficient file system from each group. Secondly, we perform an inter group comparison of the selected file systems in each group. This exercise is repeated for following factors of

5 file systems to provide best possible base for next generation file systems: 1. Storage Limits 2. Metadata 3. Characteristics 4. Allocation Strategy 3.1 INTRA GROUP COMPARISON Linux s: All the Linux file systems in consideration store file owner, last access timestamps and last metadata change timestamps. They do not store archive timestamps. All of them have POSIX file permissions, access control lists, security and extended attributes. They support hard links and symbolic links, they are case sensitive and case preservative, but they do not support file change logging, snapshot and encryption. None of the Linux file systems support tail packing, variable file-block size and transparent compression, whereas all of them support sparse files. ext4 is better than the other file systems of the ext series in terms of maximum filename length, maximum file size and maximum volume size. It records creation timestamp, last modification timestamp and checksum which the other ext file systems do not record. Both ext4 and ext3 are journaling file systems. ext4 supports extents and allocation on-flush. Therefore, among the Linux file systems the latest ext4 is superior of all. Macintosh s: Both HFS and HFS Plus records creation timestamps, last modification timestamps and last archive timestamps. Both of them support extended attributes, symbolic links and case preservation. On the other hand, they do not support checksum, snapshot and encryption. Among the Macintosh s, HFS Plus has better storage limits than HFS Standard in terms of maximum filename length, maximum file size and maximum volume size. HFS Plus records file owner, last access timestamp, last metadata change timestamp, POSIX file permissions, access control lists and security which HFS do not store. HFS is a journaling file system, it is case sensitive and supports hard links and file change log. Therefore, the newer HFS Plus is a better file system than the older HFS Standard in all respects. Windows s: FAT32, exfat and NTFS provides similar limits in case of maximum filename length. All of them record creation timestamps, last access timestamps and last modification timestamps. They do not store last archive timestamps and checksum. Moreover, they do not support tail packing, variable file-block size and allocation on-flush. NTFS file system is better than the other windows file systems in consideration in terms of maximum file size & maximum volume size. It stores file owner, last metadata change timestamp, POSIX file permissions, access control lists, security labels, extended attributes which others do not store. NTFS is a journaling file system supports hard links, symbolic links, file change log, encryption, extents, sparse files and transparent compression with partial support of snapshot. It is case sensitive and case preserving as well. Therefore, considering all the above facts it can be said that NTFS file system is superior to others in the group. Solaris s: Both VxFS and ZFS provide similar storage limits in case of maximum filename length, maximum file size and maximum volume size and record similar metadata except that ZFS stores last archive timestamps and checksum which VxFS does not. Both the Solaris file systems are journaling and support hard links, symbolic links, snapshot and sparse files. They are also case sensitive and case preservative as well. ZFS is more secure and robust and lacks only file change logging feature. VxFS supports extents whereas ZFS supports variable block size, allocation on-flush, transparent compression and tail packing partially. Though it is seen that both VxFS and ZFS provides similar set of features in most cases, ZFS takes an edge over VxFS at closer comparison. 3.2 INTER GROUP COMPARISON ext4, HFS+, NTFS and ZFS have been identified as superior file systems in terms of features within their groups. The next step would be to have a cross group comparison. From the statistics presented in Table 1 we can conclude that, in case of storage limits Windows file system NTFS and Solaris file system ZFS are at par and superior than the rest of the file systems in consideration. From the statistics presented in Table 2 we can conclude that Solaris file system ZFS stores all metadata those other file systems in consideration misses out. From the statistics presented in Table 3 we can conclude that Windows file system NTFS followed by Solaris file system ZFS tops the list of file systems in consideration in terms of features. From the statistics presented in Table 4.1 & Table 4.2 we can conclude that Solaris file system ZFS has the most efficient allocation strategies of all file systems considered for this study. Maximum filename length Maximum file size Maximum volume size ext4 256 bytes 16 GB to 16 TB 1 EB Slightly HFS+ 255 bytes less than 8 EB NTFS 255 bytes 16 EB 16 EB ZFS 255 bytes 16 EB 16 EB Table 1. Comparison based on Storage Limits Slightly less than 8 EB Last Archive Timestamp Checksum ext4 No Yes HFS+ Yes No NTFS No No ZFS Yes Yes Table 2. Comparison based on Metadata

6 Case Sensiti Change ve Log Snapshot Encryption ext4 Yes No No No HFS+ Partial Yes No No NTFS Yes Yes Partial Yes ZFS Yes No Yes Yes Table 3. Comparison based on Characteristics Sparse s Transparent compression ext4 Yes No HFS+ No No NTFS Yes Yes ZFS Yes Yes Table 4.1. Comparison based on Allocation Strategy Syste m Tail packing Variable Block Size Extents Table 4.2. Comparison based on Allocation Strategy Solaris file system ZFS supports higher storage capacities with higher storage limits, it records a lot of metadata to track all disk activities, it has got most of the characteristics which a modern disk file system should have and it has most of the efficient allocation strategies when compared to other file systems, with NTFS and VxFS not lagging very far behind. Ext4 and HFS+ are also efficient enough to cope up with all the recent advancements of the storage subsystem. In this comparative study we have performed a feature based analysis and comparison of different file systems and arrived at a conclusion that ZFS is a better file system which can provide a strong base and template for the design of next generation file systems and refinement of old ones to meet the growing requirements of users and handle huge disk capacities. Table 5. Results based on Comparison Allocation Flush ext4 No No Yes Yes HFS+ No No Yes Yes NTFS No No Yes No ZFS Partial Yes No Yes Factor Storage Limits Metadata Characteristics Allocation Strategies Efficient NTFS, ZFS, VxFS ZFS NTFS Close Competitor HFS+ HFS+, ext4 ZFS ZFS - REFERENCES [1] Dominic Giampaolo, Practical Design with the Be, Be, Inc. [2] Michael D. Dahlin, (1996), The Impact of Trends in Technology on Design, University of California, Berkeley. [3] G. A. Gibson, (1992), Redundant Disk Arrays: Reliable, Parallel Secondary Storage, ACM Distinguished Dissertations. MIT Press, Cambridge, Massachusetts. [4] Bellevue Linux Users Group, The Linux Information Project, Ext2fs Definition, [5] Lars Wizenius, Joanna Oja, Stephen Stafford and Alex Weeks, The Linux Administrator s Guide, [6] William von Hagen and Brian K. Jones, Linux Server Hacks, Volume Two. Chapter 10, Rescue, Recovery, and Repair, ISBN 10: [7] Michael K. Johnson, Red Hat s New Journaling : ext3, [8] Avantika Mathur, Mingming Cao, Suparna Bhattacharya, Adreas Dilger, Alex Tomas and Laurent Vivier, (2007), The new ext4 filesystem: current status and future plans, In Proceedings of the Linux Symposium. [9] Mac OS X Reference Library, HFS Plus Volume Format, Technical Note TN html [10] [11] Microsoft TechNet, Windows 2000 Server, FAT16 vs. FAT aspx [12] W. A. Bhat, S. M. K. Quadri, (2010), Review of FAT Data Structure of FAT32 file system, Oriental Journal of Computer Science & Technology, Volume 3, No 1 [13] Microsoft Help and Support, Description of FAT32. Article ID: Revision: [14] Ray Duncan, (1989), Design goals and implementation of the new High Performance, Microsoft s Journal. [15] Microsoft TechNet, Windows 2003 Server, Advantages of NTFS, [16] MSDN Platform SDK: s. Compression and Decompression. [17] VERITAS (TM), Administrator's Guide, Release 3.2. The Veritas, Chapter html

7 [18] Jeff Bonwick and Bill Moore. ZFS The Last Word In s. oup+zfs/docs/zfslast.pdf [19] Sun Microsystems, Inc., (2007), Solaris ZFS and Microsoft Server 2003, NTFS Performance, White paper, msft.pdf