Re: hot-add unformatted drive to RAID array automagically
On Friday 18 February 2005 12:09, Donovan Baarda <abo@minkirri.apana.org.au>
wrote:
> > A RAID-5 setup without NVRAM for write-back cache will give poor write
> > performance. If you want to write one sector to a RAID-5 array then the
> > controller has to read the original sector and the checksum sector and
> > then write to the sector and the new checksum. This makes a single write
> > become two reads and two writes (one read and re-write on each of two
> > disks). All good hardware RAID systems have NVRAM write-back caches to
> > allow write-combining on RAID-5 lines. This gives a massive performance
> > benefit.
>
> But surely writing 4 sectors (in a 4+1 RAID5) is implemented as writing
> 1 sector to each disk in parallel (4 data sectors, 1 checksum). So if
> you were using a block size of 2K (4 x 512Bytes disc sector size), you
> would get a 4 x speedup (assuming checksum calculation is negligible).
Writing one sector to each of four disks will not be much faster than writing
four contiguous sectors to one disk. In fact it may be slower on average as
the operation will not be complete until all disks have written so the time
taken for the operation will be the worst-case out of four seeks rather than
the average.
So writing to a RAID-5 is expected to always be slower than writing to a
single disk for small writes. If you write 1G of data contiguously then
RAID-5 should be a lot faster.
With NVRAM for write-back cache you get some benefit from having the journal
and Inode tables being written multiple times before the data is committed.
This can result in better write performance than a single disk.
--
http://www.coker.com.au/selinux/ My NSA Security Enhanced Linux packages
http://www.coker.com.au/bonnie++/ Bonnie++ hard drive benchmark
http://www.coker.com.au/postal/ Postal SMTP/POP benchmark
http://www.coker.com.au/~russell/ My home page
Reply to: