On 16/01/2013 05:31, Thomas Fjellstrom wrote:
On Tue Jan 15, 2013, Stan Hoeppner wrote:
On 1/14/2013 9:52 PM, Thomas Fjellstrom wrote:
...
It is working. And I can live with it as is, but it does seem like
something isn't right. If thats just me jumping to conclusions, well
thats fine then. But 600MB/s+ reads vs 200MB/s writes seems a tad off.
It's not off. As myself and others stated previously, this low write
performance is typical of RAID6, particularly for unaligned or partial
stripe writes--anything that triggers a RMW cycle.
That gets me thinking. Maybe try a test with the record test size set to the
stripe width, that would hopefully show some more accurate numbers.
Your 7-drive RAID-6 with 512K chunk will have a 2.5MB stripe width, or
stride, whichever is the correct term, on the basis of 5 data chunks.
Even still, a filesystem-level test cannot guarantee to be writing
records aligned to the array's data stripes.
If you do another benchmark, try running iostat concurrently, to see how
many reads are happening during the write tests.
At the same time, if in the real world you're doing streaming writes of
dozens of MB/s, I would expect that write caching would turn a good
proportion of the writes into full-stripe writes.
Cheers,
John.
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
