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