Hi! I'm attempting to understand the pros/cons between raid5 and raidz after
running into a performance issue with Oracle on zfs
(http://opensolaris.org/jive/thread.jspa?threadID=120703&tstart=0).
I would appreciate some feedback on what I've understood so far:
WRITES
raid5 - A FS block is written on a single disk (or multiple disks depending on
size data???)
raidz - A FS block is written in a dynamic stripe (depending on size of
data?)across n number of vdevs (minus parity).
READS
raid5 - IO count depends on how many disks FS block written to. (data crosses
two disks 2 IOs??)
raidz - A single read will span across n number of vdevs (minus parity).
(1single IO??)
NEGATIVES
raid5 - Write hole penalty, where if system crashes in the middle of a write
block update before or after updating parity - data is corrupt.
- Overhead (read previous block, read parity, update parity and write
block)
- No checksumming of data!
- Slow read sequential performance.
raidz - Bound by x number of IOPS from slowest vdev since blocks are striped.
Bad for small random reads
POSITIVES
raid5 - Good for random reads (between raid5 and raidz!) since blocks are not
striped across sum of disks.
raidz - Good for sequential reads and writes since data is striped across sum
of vdevs.
- No write hole penalty!
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