C[omp]ute

From: "andrew cooke" <andrew@...>

Date: Sat, 7 Apr 2007 09:59:57 -0400 (CLT)

I finally understand (I think) how having raid 5 affects the disk speed.

For large inputs and outputs, and for seeks, it is much faster than single
disks, increasing in speed by a factor of N (or perhaps N-1).  You can see
this by running iostat while the disks are in heavy use.  The IO rates for
the md device are much higher than the disks themselves, and the disks are
limiting the system (lots of IO wait in the CPUs).

This is also clear in bonnie++ output:

Version 1.01d       -Sequential Output- -Seq-Inp- --Random-
                    --Block-- -Rewrite- --Block-- --Seeks--
Machine        Size K/sec %CP K/sec %CP K/sec %CP  /sec %CP
quiet            7G 34986  10 15030   3 43503   4  73.6   0

(reformatted to drop the per character values)

For my cheap disks (320Gb SATA 7200 WD Caviar), those are significant
improvements over a disk in isolation, as you'd expect from striping.

However, "everyone" says how bad raid 5 is for writing, so why are the
write speeds so close to read?  Because, I think, the file being written
is much bigger than the 128K block size, so there's no need to read the
parity information when writing - the system is simply writing a new
parity block along with new data blocks.

This may be reflected in the lower "rewrite" figure, which is dirtying
data within a single block.

Andrew