Re: best base / worst case RAID 5,6 write speeds

[Phil Turmel <philip@turmel.org>]

On 12/10/2015 03:09 PM, Dallas Clement wrote:
> On Thu, Dec 10, 2015 at 2:06 PM, Phil Turmel <philip@turmel.org> wrote:

>> Where'd you get the worst case formulas?
> 
> Google search I'm afraid.  I think the assumption for RAID 5,6 worst
> case is having to read and write the parity + data every cycle.

Well, it'd be a lot worse than half, then.  To use the shortcut in raid5
to write one block, you have to read it first, read the parity, compute
the change in parity, then write the block with the new parity.  That's
two reads and two writes for a single upper level write.  For raid6, add
read and write of the Q syndrome, assuming you have a kernel new enough
to do the raid6 shortcut at all.  Three reads and three writes for a
single upper level write.  In both cases, add rotational latency to
reposition for writing over sectors just read.

Those RMW operations generally happen to small random writes, which
makes the assertion for sequential writes odd.  Unless you delay writes
or misalign or inhibit merging, RMW won't trigger except possibly at the
beginning or end of a stream.

That's why I questioned O_SYNC when you were using a filesystem: it
prevents merging, and forces seeking to do small metadata writes.
Basically turning a sequential workload into a random one.

Phil