Re: *terrible* direct-write performance with raid5

On Tue, 22 Feb 2005, Michael Tokarev wrote:

When debugging some other problem, I noticied that
direct-io (O_DIRECT) write speed on a software raid5
is terrible slow.  Here's a small table just to show
the idea (not numbers by itself as they vary from system
to system but how they relate to each other).  I measured
"plain" single-drive performance (sdX below), performance
of a raid5 array composed from 5 sdX drives, and ext3
filesystem (the file on the filesystem was pre-created
during tests).  Speed measurements performed with 8Kbyte
buffer aka write(fd, buf, 8192*1024), units a Mb/sec.

with O_DIRECT you told the kernel it couldn't cache anything... you're 
managing the cache.  you should either be writing 64KiB or you should 
change your chunksize to 8KiB (if it goes that low).

-dean