Checksum the individual RAID-5 blocks, rather than the entire stripe?
Depending on your the number of drives in your RAID-Z, this will
increase your metadata size by N-1 * 32 bytes. Would this not be an
undesirable cost increase on the metadata size?
In more detail: Allow the pointer to the block to contain one
checksum per device used (the count will vary if you're using a
RAID-Z style algorithm). Checksum each device's data independently,
so the pointer looks like one of:
(a) array of <device, offset, checksum> tuples
(b) <device, offset> tuple and array of checksums
The former is really nice, as it also allows you to place the block
arbitrarily within a disk. This could potentially allow a more
efficient implementation of RAID-Z with variable sized disks (some
disks could effectively be concatenated as in a RAID-0 reducing the
overall striped width).
Writing a partial stripe is more interesting. With style (b) block
pointers -- RAID-Z style -- you need to do a read/modify/write of
the stripe to get all the new data into the right place. But with
style (a) block pointers, you’re back to RAID-5 style writes (read
old data+parity or remaining data, write new data+parity). (You
don't need to rewrite the whole stripe since the block pointer can
refer to the original partial stripe.)
Indeed!
Out of interest what are the major concerns with increasing the block
pointer size from 128 bytes?
James_______________________________________________
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