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8542ef852a
Authored by: Matt Ahrens <mahrens@delphix.com> Reviewed by: Saso Kiselkov <saso.kiselkov@nexenta.com> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: George Melikov <mail@gmelikov.ru> Reviewed-by: Don Brady <don.brady@intel.com> Ported-by: Matt Ahrens <mahrens@delphix.com> RAID-Z requires that space be allocated in multiples of P+1 sectors, because this is the minimum size block that can have the required amount of parity. Thus blocks on RAIDZ1 must be allocated in a multiple of 2 sectors; on RAIDZ2 multiple of 3; and on RAIDZ3 multiple of 4. A sector is a unit of 2^ashift bytes, typically 512B or 4KB. To satisfy this constraint, the allocation size is rounded up to the proper multiple, resulting in up to 3 "pad sectors" at the end of some blocks. The contents of these pad sectors are not used, so we do not need to read or write these sectors. However, some storage hardware performs much worse (around 1/2 as fast) on mostly-contiguous writes when there are small gaps of non-overwritten data between the writes. Therefore, ZFS creates "optional" zio's when writing RAID-Z blocks that include pad sectors. If writing a pad sector will fill the gap between two (required) writes, we will issue the optional zio, thus doubling performance. The gap-filling performance improvement was introduced in July 2009. Writing the optional zio is done by the io aggregation code in vdev_queue.c. The problem is that it is also subject to the limit on the size of aggregate writes, zfs_vdev_aggregation_limit, which is by default 128KB. For a given block, if the amount of data plus padding written to a leaf device exceeds zfs_vdev_aggregation_limit, the optional zio will not be written, resulting in a ~2x performance degradation. The problem occurs only for certain values of ashift, compressed block size, and RAID-Z configuration (number of parity and data disks). It cannot occur with the default recordsize=128KB. If compression is enabled, all configurations with recordsize=1MB or larger will be impacted to some degree. The problem notably occurs with recordsize=1MB, compression=off, with 10 disks in a RAIDZ2 or RAIDZ3 group (with 512B or 4KB sectors). Therefore this problem has been known as "the 1MB 10-wide RAIDZ2 (or 3) problem". The problem also occurs with the following configurations: With recordsize=512KB or 256KB, compression=off, the problem occurs only in rarely-used configurations: * 4-wide RAIDZ1 with recordsize=512KB and ashift=12 (4KB sectors) * 4-wide RAIDZ2 (either recordsize, either ashift) * 5-wide RAIDZ2 with recordsize=512KB (either ashift) * 6-wide RAIDZ2 with recordsize=512KB (either ashift) With recordsize=1MB, compression=off, ashift=9 (512B sectors) * RAIDZ1 with 4 or 8 disks * RAIDZ2 with 4, 8, or 10 disks * RAIDZ3 with 6, 8, 9, or 10 disks With recordsize=1MB, compression=off, ashift=12 (4KB sectors) * RAIDZ1 with 7 or 8 disks * RAIDZ2 with 4, 5, or 10 disks * RAIDZ3 with 6, 9, or 10 disks With recordsize=2MB and larger (which can only be selected by changing kernel tunables), many configurations are affected, including with higher numbers of disks (up to 18 disks with recordsize=2MB). Increase zfs_vdev_aggregation_limit to allow the optional zio to be aggregated, thus eliminating the problem. Setting it to 256KB fixes all commonly-used configurations. The solution is to aggregate optional zio's regardless of the aggregation size limit. Analysis sponsored by Intel Corp. OpenZFS-issue: https://www.illumos.org/issues/8005 OpenZFS-commit: https://github.com/openzfs/openzfs/pull/321 Closes #5931 |
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