raid 1 expected performance

[Eric Rostetter]

Quoting michalwd1979 <michalwd1979 at o2.pl>:

> The HDDs are quite old, raw writing speed is about 47MB/s to each  
> (checked it before installing RAID on normal SCSI) reading about  
> 60MB/s. Now with RAID 1, lvm and XFS filesystem I have about  
> 35-38MB/s writing speed. Is this normal?

Unless you cheat (have it ack a write when it hits cache instead of disk)
you will always see slower write performance with LVM+RAID1 since it
has LVM overhead and since RAID1 must write to both disks before it
completes.  It always takes longer to write to two disks than to one,
how much longer depends on the disk load.

However, you should be able to get faster read times (since you have
twice as many spindles, and it only has to read from one of the two
disks).

Generally, RAID 1 means slower writes and faster reads than a single disk.
And throwing in something like LVM will just add a bit more overhead, and
hence a bit more slowness (hardly noticable usually, but mentioned for
completeness).

> I assume that I will not do anything about old slow hdds but if the  
> performance drop after installing raid 1 and filesystem is normal?

Yes.  You may be able to speed it up if that raid card supports cache.
Or you might even get faster results with software raid, if you have
sufficient free CPU.  But that would depend on your needs.

I would say that dropping from 60 to 40 MB/sec isn't so bad, for the
added safety it provides...

> Regards,
> Michael

-- 
Eric Rostetter
The Department of Physics
The University of Texas at Austin

Go Longhorns!