Re: RAID-1 to RAID-5 online migration?
On Mon, 13 Sep 2004 09:55, Donovan Baarda <abo@minkirri.apana.org.au> wrote:
> > Do you have benchmark results to support this assertion? Last time I
> > tested the performance of software RAID-1 on Linux I was unable to get
> > anywhere near 2x disk speed for writing. I did tests by reading two
> > files that were 1G in size and the operation took considerably longer
> > than reading a single 1G file from a non-RAID system. If RAID-1 was
> > delivering twice the read throughput then I should be able to read two 1G
> > files concurrently from a RAID-1 in the same time as would be taken to
> > read a single 1G file from a single disk.
>
> I think Russel must be checking if the class is awake :-)
>
> That doesn't sound like a fair test; reading two files at once means the
> heads have to bounce around all over the place.
No it doesn't! In an ideal situation each read request would go to the disk
who's head was nearest to the requested data. If a program is reading data
that is sequential on disk (mostly the case if you copy large files onto a
file system that otherwise has no writes) then each read request should be
sent to the same disk.
The result should be that each disk performs sequential reads of a single file
with good performance.
> If you are just talking throughput, then reading a 1G file should take
> half the time on a RAID-1 that it does on a single disk.
No. If the 1G file is contiguous then having a single disk read through it
all will give the maximum possible speed. Using two disks to increase
performance of reading a single large file requires either RAID-0 or very
large read buffers. Linux does not seem to have such large read buffers.
Test it out.
> I suspect that reading 2 1G files at once on RAID-1 will be not much
> faster than reading 2 1G files on a single disk, because reading two
> files at once will probably be seek-bound, not throughput bound. RAID-1
> boosts throughput, not latency.
It should do so, but it doesn't seem to do it very well in Linux software
RAID. Do a test of reading two 1G files from a RAID-1 and I expect that
you'll find that nothing has changed since the last time I tested and that
performance is much less than you would hope for.
> HDD latency is a killer. It is significantly faster to read small
> objects from another machine's RAM over ethernet than off the local HDD;
> HDD latency is ~10ms, ethernet is <1ms.
Yes, in many situations NFS can outperform a local disk.
--
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