Introduction to Linux features for disk I/O

Transcription

1 Martin Kammerer 3/22/11 Introduction to Linux features for disk I/O visit us at Linux on System z Performance Evaluation

2 Considerations for applications decision when and for which data to make I/O system calls think of keeping frequently used file content in user data space or if needed in shared memory this is the best place where the data should be cached consider the right size of the I/O request. It may be better to issue a few larger requests, containing more data than issuing many I/O requests with little data. 2 Introduction to Linux features for disk I/O

3 Linux kernel components involved in disk I/O Application program Issuing reads and writes Virtual File System (VFS) Logical Volume Manager (LVM) Multipath Device mapper (dm) Block device layer Page cache I/O scheduler VFS dispatches requests to different devices and Translates to sector addressing LVM defines the physical to logical device relation Multipath sets the multipath policies dm holds the generic mapping of all block devices and performs 1:n mapping for logical volumes and/or multipath Page cache contains all file I/O data, direct I/O bypasses the page cache I/O schedulers merge, order and queue requests, start device drivers via (un)plug device Data transfer handling Direct Acces Storage Device driver 3 Introduction to Linux features for disk I/O Device drivers Small Computer System Interface driver (SCSI) z Fiber Channel Protocol driver (zfcp) Queued Direct I/O driver (qdio)

4 Logical volumes Linear logical volumes allow an easy extension of the file system Striped logical volumes Provide the capability to perform simultaneous I/O on different stripes Allow load balancing Are also extendable May lead to smaller I/O request sizes than linear or no logical volumes Don't use logical volumes for / or /usr If the logical volume gets corrupted your system is gone Logical volumes require more CPU cycles than physical disks Consumption increases with the number of physical disks used for the logical volume 4 Introduction to Linux features for disk I/O

5 Linux multipath connects a volume over multiple independent paths and/or ports multibus mode for load balancing to increase bandwidth limitations of a single path uses all paths in a priority group in a round-robin manner after a configurable amount of I/Os the path is switched See rr_min_io in multipath.conf method to increase throughput for ECKD volumes with static PAV devices in Linux distributions older than SLES11 and RHEL6 SCSI volumes in all Linux distributions failover mode for high availability the volume remains reachable in case of a single point of failure in the connecting hardware should one path fail, the operating system will route I/O through one of the remaining paths, with no changes visible to the applications will not increase performance 5 Introduction to Linux features for disk I/O

6 Page cache good or bad? the page cache helps Linux to economize I/O requires Linux memory pages changes its size depending on how much memory is used by processes writes are delayed and data in the page cache can have multiple updates before being written to disk. write requests in the page cache can be merged to larger I/O requests reads can be made faster by adding a read ahead quantity, depending on the historical behavior of file system accesses by applications consider the settings of the configuration parameters swappiness, dirty_ratio, dirty_background ratio Linux does not know which data the application really needs next. It makes only a guess 6 Introduction to Linux features for disk I/O

7 Page cache administration /proc/sys/vm/swappiness Is one of several parameters to influence the swap tendency Can be set individually with percentage values from 0 to 100 The higher the swappiness value, the more the system will swap A tuning knob to let the system prefer to shrink the Linux page cache instead of swapping processes Different kernel levels may react differently on the same swappiness value pdflush These processes write data out of the page cache /proc/sys/vm/dirty_background_ratio controls if writes are done more consistently to the disk or more in a batch /proc/sys/vm/dirty_ratio If this limit is reached writing to disk needs to happen immediately Both can be set individually with percentage values from 0 to Introduction to Linux features for disk I/O

8 Consider to use... direct I/O: bypasses the page cache is a good choice in all cases where the application does not want Linux to economize I/O and/or where the application buffers larger amount of file contents async I/O: prevents the application from being blocked in the I/O system call until the I/O completes allows read merging by Linux in case of using page cache can be combined with direct I/O temporary files: should not reside on real disks, a ram disk or tmpfs allows fastest access to these files they don't need to survive a crash, don't place them on a journaling file system file system: use ext3 and select the appropriate journaling mode (journal, ordered, writeback) turning off atime is only suitable if no application makes decisions on "last read" time, consider relatime instead 8 Introduction to Linux features for disk I/O

9 Linux under z/vm Minidisk cache requires extra storage in VM, can be in central or in expanded storage or both has a fixed size contains data from all guest systems who have this option set on can be great for shared minidisks with significant read I/O not usable for SCSI LUNs or dedicated ECKD dasd VM does not know which data the Linux guests really need next For very I/O intense Linux guests consider to use dedicated devices move to LPAR 9 Introduction to Linux features for disk I/O

10 Storage server provide a big cache to avoid real disk I/O provide a Non Volatile Storage (NVS) to ensure that cached data, which has not yet been destaged to disk devices is not lost in case of a power outage provide algorithms to optimize disk access and cache usage, optimizes for sequential I/O or for random I/O have the biggest challenge with heavy random write, because this is limited by the destage speed if the NVS is full 10 Introduction to Linux features for disk I/O

11 DS8000 storage server - ranks The smallest unit is a array of disk drives A RAID 5 or RAID 10 technique is used to increase the reliability of the disk array, defining a rank The volumes for use by the host systems, are configured on the rank The volume has stripes on each disk drive All configured volumes will share the same disk drives volume rank RAID technology Disk drives 11 Introduction to Linux features for disk I/O

12 DS8000 storage server components A rank represents a raid array of disk drives is connected to the servers by device adapters belongs to one server for primary access A device adapter connects ranks to servers Each server controls half of the storage servers disk drives, read cache and NVS Read cache NVS Server 0 Device Adapter volumes ranks Disk drives Read cache Server NVS 8 12 Introduction to Linux features for disk I/O

13 DS8000 storage server extent pool Extent pools consist of one or several ranks can be configured for one type of volumes only: either ECKD dasds or SCSI LUNs are assigned to one server Some possible extent pool definitions for ECKD and SCSI are shown in the example Server 0 Device Adapter volumes ranks ECKD ECKD ECKD 4 SCSI 5 SCSI Server SCSI SCSI 8 ECKD 13 Introduction to Linux features for disk I/O

14 DS8000 storage pool striped volume A storage pool striped volume (rotate extents) is defined on a extent pool consisting of several ranks It is striped over the ranks in stripes of 1 GiB volume ranks 1 As shown in the example The 1 GiB stripes are rotated over all ranks The first storage pool striped volume starts in rank 1, the next one would start in rank Disk drives 14 Introduction to Linux features for disk I/O

15 DS8000 storage pool striped volume A storage pool striped volume uses disk drives of multiple ranks uses several device adapters is bound to one server Device Adapter volumes ranks 1 Server 0 extent pool 2 3 Server 1 Disk drives 15 Introduction to Linux features for disk I/O

16 DS8000 striped volumes comparison (1) maximum number of ranks for the constructed volumetotal number of ranks can be challenging, e.g. several administrating disks within Linux hundred for a database simple volume extendable? yes no maximum I/O request size DS8000 setup for Linux LVM striping DS8000 storage pool striping striping is done in Linux storage server disk placement need to plan for don't care take care of picking subsequent effort to construct the volume disks from different ranks configure storage server Volume = 1 disk, SCSI unlimited, Lv = many disks SCSI 20 GiB to e.g. 500 GiB, ECKD max. 180 usual disk sizes extent pool 50 GiB, ECKD mod27 or mod54 1 rank GiB (EAV) multiple ranks multipathing stripe size (e.g. 64 KiB) SCSI: multipath multibus or multipath failover ECKD: path group Total number of one server side (50%) maximum provided by the device driver (e.g. 512 KiB) SCSI: multipath multibus, ECKD: path group 16 Introduction to Linux features for disk I/O

17 DS8000 striped volumes comparison (2) Equivalent resource usage with storage pool striped volumes volumes from single rank extents Storage pool striping allows consolidating the space of several smaller volumes of single rank extents in one big volume For volumes from single rank extents the operating system must take care of the striping Server 0 Device Adapter volumes extent pool ranks Server Introduction to Linux features for disk I/O

18 XIV storage server Volumes can't be placed in single ranks (i.e. one single interface module or data module) Offers only storage pool striping Stripe size is 1 MiB The volumes in the storage pool can use the entire storage server Currently max. 4 Gbps FCP links Provides only SCSI volumes Recommendation for Linux Use multipath multibus to access the volumes 18 Introduction to Linux features for disk I/O

19 Disk I/O attachment and DS8000 storage subsystem Each disk is a configured volume in a extent pool (here extent pool = rank) Host bus adapters connect internally to both servers Channel path 1 Channel path 2 Channel path 3 Channel path 4 Channel path 5 Channel path 6 Channel path 7 Channel path 8 Switch Host Bus Adapter 1 Host Bus Adapter 2 Host Bus Adapter 3 Host Bus Adapter 4 Host Bus Adapter 5 Host Bus Adapter 6 Host Bus Adapter 7 Host Bus Adapter 8 Server 0 Server 1 Device Adapter ranks ECKD SCSI ECKD SCSI ECKD SCSI ECKD FICON Express 8 SCSI 19 Introduction to Linux features for disk I/O

20 Connection to the storage server FICON Express4, or 8. Consider that not all ports from the same feature can be used simultaneously at full speed on the host side and on the storage server side Consider to use switches which do not limit the bandwidth of the channels System z Switch DS8K FICON Port FCP Port Not used 20 Introduction to Linux features for disk I/O

21 General layout for FICON/ECKD Application program VFS LVM Multipath dm Block device layer Page cache I/O scheduler dasd driver The dasd driver starts the I/O on a subchannel Each subchannel connects to all channel paths in the path group Each channel connects via a switch to a host bus adapter A host bus adapter connects to both servers Each server connects to its ranks Channel subsystem a b c d chpid 1 chpid 2 chpid 3 chpid 4 Switch HBA 1 HBA 2 HBA 3 HBA 4 Server 0 Server 1 DA ranks Introduction to Linux features for disk I/O

22 FICON/ECKD dasd I/O to a single disk Application program VFS Assume that subchannel a corresponds to disk 2 in rank 1 The full choice of host adapters can be used Only one I/O can be issued at a time through subchannel a All other I/Os need to be queued in the dasd driver and in the block device layer until the subchannel is no longer busy with the preceding I/O DA ranks Block device layer Page cache I/O scheduler dasd driver Channel subsystem a! chpid 1 chpid 2 chpid 3 Switch HBA 1 HBA 2 HBA 3 Server chpid 4 HBA 4 Server Introduction to Linux features for disk I/O

23 FICON/ECKD dasd I/O to a single disk with PAV (SLES10 / RHEL5) Application program VFS VFS sees one device The device mapper sees the real device and all alias devices Parallel Access Volumes solve the I/O queuing in front of the subchannel Each alias device uses its own subchannel Additional processor cycles are spent to do the load balancing in the device mapper The next slowdown is the fact that only one disk is used in the storage server. This implies the use of only one rank, one device adapter, one server Multipath dm Block device layer Page cache I/O scheduler dasd driver Channel subsystem a chpid 2 chpid 3 chpid 4 Switch HBA 1 HBA 2 HBA 3 HBA 4 Server 0 Server 1 DA!! b c d chpid 1 ranks Introduction to Linux features for disk I/O

24 FICON/ECKD dasd I/O to a single disk with HyperPAV (SLES11 / RHEL6) Application program VFS VFS sees one device The dasd driver sees the real device and all alias devices Load balancing with HyperPAV and static PAV is done in the dasd driver. The aliases need only to be added to Linux. The load balancing works better than on the device mapper layer. Less additional processor cycles are needed than with Linux multipath. The next slowdown is the fact that only one disk is used in the storage server. This implies the use of only one rank, one device adapter, one server Block device layer Page cache I/O scheduler dasd driver Channel subsystem a b c d chpid 1 chpid 2 chpid 3 chpid 4 Switch HBA 1 HBA 2 HBA 3 HBA 4 Server 0 Server 1 DA! ranks Introduction to Linux features for disk I/O

25 FICON/ECKD dasd I/O to a linear or striped logical volume Application program VFS LVM dm! Block device layer Page cache I/O scheduler dasd driver VFS sees one device (logical volume) The device mapper sees the logical volume and the physical volumes Additional processor cycles are spent to map the I/Os to the physical volumes. Striped logical volumes require more additional processor cycles than linear logical volumes With a striped logical volume the I/Os can be well balanced over the entire storage server and overcome limitations from a single rank, a single device adapter or a single server To ensure that I/O to one physical disk is not limited by one subchannel, PAV or HyperPAV should be used in combination with logical volumes Channel subsystem a b c d chpid 1 chpid 2 chpid 3 chpid 4 Switch HBA 1 HBA 2 HBA 3 HBA 4 Server 0 Server 1 DA ranks Introduction to Linux features for disk I/O

26 FICON/ECKD dasd I/O to a storage pool striped volume with HyperPAV (SLES11) Application program VFS A storage pool striped volume makes only sense in combination with PAV or HyperPAV VFS sees one device The dasd driver sees the real device and all alias devices The storage pool striped volume spans over several ranks of one server and overcomes the limitations of a single rank and / or a single device adapter Storage pool striped volumes can also be used as physical disks for a logical volume to use both server sides. To ensure that I/O to one dasd is not limited by one subchannel, PAV or HyperPAV should be used Block device layer Page cache I/O scheduler dasd driver Channel subsystem a b c d chpid 1 chpid 2 chpid 3 chpid 4 Switch HBA 1 HBA 2 HBA 3 HBA 4! Server 0 Server 1 DA ranks extent pool Introduction to Linux features for disk I/O

27 General layout for FCP/SCSI Application program VFS LVM Multipath dm Block device layer Page cache I/O scheduler SCSI driver zfcp driver qdio driver 27 Introduction to Linux features for disk I/O The SCSI driver finalizes the I/O requests The zfcp driver adds the FCP protocol to the requests The qdio driver transfers the I/O to the channel A host bus adapter connects to both servers Each server connects to its ranks chpid 5 chpid 6 chpid 7 chpid 8 Switch HBA 5 HBA 6 HBA 7 HBA 8 Server 0 Server 1 DA ranks

28 FCP/SCSI LUN I/O to a single disk Application program VFS Assume that disk 3 in rank 8 is reachable via channel 6 and host bus adapter 6 Up to 32 (default value) I/O requests can be sent out to disk 3 before the first completion is required The throughput will be limited by the rank and / or the device adapter There is no high availability provided for the connection between the host and the storage server DA ranks Block device layer Page cache I/O scheduler SCSI driver zfcp driver qdio driver 28 Introduction to Linux features for disk I/O! chpid 5 chpid 6 chpid 7 chpid 8 Switch HBA 5 HBA 6 HBA 7 HBA 8 Server 0 Server 1!

29 FCP/SCSI LUN I/O to a single disk with multipathing Application program VFS VFS sees one device The device mapper sees the multibus or failover alternatives to the same disk Administrational effort is required to define all paths to one disk Additional processor cycles are spent to do the mapping to the desired path for the disk in the device mapper Multipath dm! Block device layer Page cache I/O scheduler SCSI driver zfcp driver qdio driver 29 Introduction to Linux features for disk I/O chpid 5 chpid 6 chpid 7 chpid 8 Switch HBA 5 HBA 6 HBA 7 HBA 8 Server 0 Server 1 DA! ranks

30 FCP/SCSI LUN I/O to a linear or striped logical volume Application program VFS LVM dm Block device layer Page cache I/O scheduler SCSI driver zfcp driver qdio driver 30 Introduction to Linux features for disk I/O! VFS sees one device (logical volume) The device mapper sees the logical volume and the physical volumes Additional processor cycles are spent to do map the I/Os to the physical volumes. Striped logical volumes require more additional processor cycles than linear logical volumes With a striped logical volume the I/Os can be well balanced over the entire storage server and overcome limitations from a single rank, a single device adapter or a single server To ensure high availability the logical volume should be used in combination with multipathing chpid 5 chpid 6 chpid 7 chpid 8 Switch HBA 5 HBA 6 HBA 7 HBA 8 Server 0 Server 1 DA ranks

31 FCP/SCSI LUN I/O to a storage pool striped volume with multipathing Application program VFS Storage pool striped volumes make no sense without high availability VFS sees one device The device mapper sees the multibus or failover alternatives to the same disk The storage pool striped volume spans over several ranks of one server and overcomes the limitations of a single rank and / or a single device adapter Storage pool striped volumes can also be used as physical disks for a logical volume to make use of both server sides. Multipath dm! Block device layer Page cache I/O scheduler SCSI driver zfcp driver qdio driver 31 Introduction to Linux features for disk I/O chpid 5 chpid 6 chpid 7 chpid 8 Switch HBA 5 HBA 6 HBA 7 HBA 8 Server 0 Server 1! DA ranks extent pool 8

32 Summary - I/O processing characteristics FICON/ECKD: 1:1 mapping host subchannel:dasd Serialization of I/Os per subchannel I/O request queue in Linux Disk blocks are 4 KiB High availability by FICON path groups Load balancing by FICON path groups and Parallel Access Volumes FCP/SCSI Several I/Os can be issued against a LUN immediately Queuing in the FICON Express card and/or in the storage server Additional I/O request queue in Linux Disk blocks are 512 Bytes High availability by Linux multipathing, type failover or multibus Load balancing by Linux multipathing, type multibus 32 Introduction to Linux features for disk I/O

33 Summary - Performance considerations to speed up the data transfer to one or a group of volumes use more than 1 rank, because then simultaneously more than 8 physical disks are used more cache and NVS can be used more device adapters can be used use more than 1 FICON Express channel in case of ECKD use more than 1 subchannel per volume in the host channel subsystem High Performance FICON and/or Read Write Track Data speed up techniques use more than 1 rank: storage pool striping, Linux striped logical volume use more channels: ECKD logical path groups, SCSI Linux multipath multibus ECKD use more than 1 subchannel: PAV, HyperPAV 33 Introduction to Linux features for disk I/O

34 Trademarks IBM, the IBM logo, and ibm.com are trademarks or registered trademarks of International Business Machines Corp., registered in many jurisdictions worldwide. Other product and service names might be trademarks of IBM or other companies. A current list of IBM trademarks is available on the Web at Copyright and trademark information at Linux is a registered trademark of Linus Torvalds in the United States, other countries, or both. Java and all Java-based trademarks and logos are trademarks of Sun Microsystems, Inc. in the United States, other countries, or both. Other product and service names might be trademarks of IBM or other companies. Source: If applicable, describe source origin 34 Introduction to Linux features for disk I/O

35 Backup slides 35 Introduction to Linux features for disk I/O

36 Monitoring disk I/O Merged requests I/Os per second Throughput Request size Queue length Service times utilization Output from iostat -dmx 2 /dev/sda Device: rrqm/s wrqm/s r/s w/s rmb/s wmb/s avgrq-sz avgqu-sz await svctm %util sda sequential write sda sequential write sda random write sda sequential read sda random read 36 Introduction to Linux features for disk I/O