[wellylug] preliminary SATA Linux software RAID0/RAID1 compar ison

Peter Jones PeterJ at indeserve.co.nz
Wed Oct 13 12:36:14 NZDT 2004


-----Original Message-----
From: Brent Wood [mailto:pcreso at pcreso.com]
Sent: Wednesday, 13 October 2004 12:22 p.m.
To: wellylug at lists.wellylug.org.nz
Subject: Re: [wellylug] preliminary SATA Linux software RAID0/RAID1
comparison

Um... Given that the RAID1 we are talking about is implemented via software,
the OS still needs to run the code to write it twice, once to each disk. No
difference there whether the drives are on the same or different channels.

With hardware RAID1, the OS writes once to the controller, which then writes
to
both disks. So with software RAID1, the cpu overhead is likely to be greater
than with a hardware implementation. Probably somewhere less than double.

With RAID0, writes are to two disks, but 'tis only actually written once. So
some cpu load in sorting out which data goes to what disk, but only the one
write. So half the actual data volume to shift.

I agree that the hardware will generally respond faster to the OS
instructions
if the drives are on separate channels, that's why I'm using SATA drives.


What I'm looking at doing to get the best of all worlds, is a 20+20Gb RAID0
for
a working area, giving 40Gb of very fast read filesystem for large files.
Add a
20+20 RAID1 for a secure data repository. I move files to the RAID0 when I'm
using them, & copy back if updated or just delete if unchanged. 

Postgres database is on the RAID0, with a cron database dump bzip2'd to the
RAID1 partition nightly.

It will involve a bit of hands on data/file management to get the best out
of
it, but should give me performance where I need it with a better level of
data
security than just striping.

If this works well, and if I/O is still a problem, I have the option (if I
can
justify the $$) of moving to 2x WD 74Gb Raptor drives, which, looking at the
benchmarks, should boost I/O by at least 50%.

I just need a few days to play & see if I'm memory, cpu or I/O bound. Some
jobs
are still gonna take days, just fewer of them.


As usual, any ideas/suggestions to help tweak it (within budget :-)
appreciated...


   Brent

Are SCSI disks and a hardware controller not an option?

I know that the (old but very good) DPT SmartCache IV controllers play
nicely with Linux.

Peter


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