[zfs-discuss] MySQL Performance Blog about ZoL

Kudinov Andrey elhana at sigil-guild.org
Thu May 30 16:19:38 EDT 2013


Mirrored Performance Considerations

When writing to mirrored vdevs, ZFS will write in parallel to all of
the mirror disks at once and return from the write operation when all
of the individual disks have finished writing. This means that for
writes, each mirrored vdev will have the same performance as its
slowest disk. This is true for both IOPS (number of IO operations per
second) and bandwidth (MB/s).

When reading from mirrored vdevs, ZFS will read blocks off the
mirror's individual disks in a round-robin fashion, thereby increasing
both IOPS and bandwidth performance: You'll get the combined aggregate
IOPS and bandwidth performance of all disks.

But from the point of view of a single application that issues a
single write or read IO, then waits until it is complete, the time it
will take will be the same for a mirrored vdev as for a single disk:
Eventually it has to travel down one disk's path, there's no shortcut
for that.

TLDR: ZFS doesn't read every block from both discs in the mirror,
there is checksum to verify that data is valid.

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