Megaraid with brakes on?

Martin Sarsale martin at malditainternet.com
Wed Jan 3 20:08:55 CST 2007


Dan,

> I'm not sure how good the ext3 file system is with loads like that;
> you might also see improvement with another filesystem.  This is worthy
> of  investigation.  Also, depending on how random your workload really
> is you may see improvement by tuning the kernel's metadata buffering
> (inode & directory caches; i'm not sure exactly what is available).

can you give me more info about meta data buffering? some keywords
would be great.

>
> You also appear to be doing a non-trivial number of writes.  small
> writes (log files?) to a RAID5 device really slow down things; the RAID
> controller needs to first read the existing stripe, then write the entire
> new stripe after merging the newly written data and recalculating the
> RAID5 parity.
>
> Here's how I would approach this:
>
> Assuming that you have lots of simultaneous requests, you would probably
> see significant improvement by adding a fourth 73G disk, converting the
> system to use a pair of hardware RAID1 devices, and doing a software
> concatenation of the two RAID1's (or, just segmenting your data across
> two separate filesystems).
>
> alternately, you could do a RAID10 in hardware; but generally adding
> RAID0 is useful to increase raw throughput on large files.  You don't
> need that, and my guess is that it would be slower than the concatenated
> RAID1's for your workload.
>
> As long as you're taking downtime to do this, you might try adding a
> third pair of disks in another RAID1.   i'm not sure they'll fit in
> that chassis, and if it did it would leave you without the ability to
> have a hot-spare.


We're changing servers (not related to this issue) and we'll build a
RAID10 in the new ones. What do you think it's better, assuming we
want to have all the disks in a RAID?
Hardware RAID0 and Software RAID1
Software RAID0 and Hardware RAID1
or Hardware RAID10 ?

thanks a lot!
-- 
Martin Sarsale - martin at malditainternet.com



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