[CentOS] Shrinking a volume group
alex at milivojevic.org
Wed Sep 13 19:41:18 UTC 2006
Quoting Steve Bergman <steve at rueb.com>:
> OK. Now I'm a bit confused. Raid 1 read performance is not what I
Note that hdparm reports the raw speed of the drive. In real world,
you will at least have overhead of the file system. Possibly also
overhead of LVM and md device drivers (if you use them).
libata in current stable kernels doesn't support NCQ. The support for
NCQ will be added in 2.6.18 (currently at rc7 level). Unless Red Hat
bumps the kernel version in final release of RHEL5 or backports NCQ to
2.6.17 (I wouldn't bet on backporting, there were some major changes
in libata), you are not going to see it in forthcomming RHEL5 either
(beta 1 uses 2.6.17 kernel).
If you run only a single process, md device driver will read from
disks in round robin fashion. You can even observe this visually if
your hard drives have separate LEDs. Only one of them will be active
at any point in time during sequential read test. It's not smart
enough to stripe reads on RAID1. I'm not sure if this is due to lack
of NCQ support and how much (if) it will help once NCQ support is
added to the linux kernel. I didn't have any spare SCSI system to
test how things work there.
However, if you run two processes, md driver will do reads from
different drives in parallel. Again, you can observe this visually if
your drives have individual LEDs (both will be lit).
I've run couple of benchmarks (using bonnie++), that also show this
numerically. For "one drive test" I simply detached second disk from
the mirror, so the overhead of drivers (md+lvm+ext3) is about the
same. The numbers for "two processes" tests are for single process
(multiply by two to get total throughput).
Test seq write (kB/s) seq read (kB/s)
raid1, single process 37716 47330
raid1, two processes (each) 16570 31572
degraded raid1, single process 39076 47627
degraded raid1, two processes (each) 16368 6759
Writing to single drive (degraded RAID1 in this case) is a bit faster
than writing to RAID1, since there's no need to wait for data to be
written to both drives.
If two processes are writing to the same disk, it's about the same.
Note that 16.5MB/s is per process (total for disk is 33MB/s). So we
have 33MB/s vs. 37MB/s). I'd expect bigger difference due to all the
extra disk seeks. So this result is actually very good.
You can see effect of md driver not striping reads on RAID1. Almost
the same speed (47MB/s) for RAID1 and degraded-RAID1 (single drive)
On the other hand, if there are two processes reading in parallel,
each is able to read 31.5 MB/s, which totals to 63MB/s. Much better.
It's still not double the speed (those two processes are fighting for
system resources after all, not only disks but also CPU time).
Two processes reading from degraded RAID-1 clearly sucks. Total
throughput drops to aruond 13-ish MB/s. I've no good explanation for
such a low number (it is less than a half of the write throughput).
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