Re: Questions about software RAID level 1 (how long should it take to sync, etc?)

[Matt Cuttler <mcuttler@bnl.gov>]

> 
> The min and max rebuild rates are in /proc/sys/dev/raid.  The
> RAID subsystem will *try* to rebuild at a rate between these
> limits.  Depending upon hardware, you might typically achieve
> 10-20 MBps on a lightly loaded system.  It can easily take 5
> hours or more to sync 160GB.
> 
> You're only seeing 1.6MBps.  You may have your max rate set
> too low or you may have slow drives or maybe (U)DMA disabled.
> 


1.6MBytes/sec does seem slow, even for simple PATA drives.

(not exactly comparing apples to apples, but..) I recently created a
RAID5 array on a 2.4GHz Xeon, consisting of 5x 146GB U320/SCSI drives.
Sync time was about 2.5 hours.

I'd be interested in seeing the output of:
'hdparm -i' and 'hdparm -tT' on /dev/hda and /dev/hdc
on this box. Do you have slaves attached to these IDE busses? Other than
that, the normal IDE troubleshooting and tweaking would apply (e.g. do
you have good, short, 80 conductor cables?).

But to answer some of your other questions (IDE gurus, feel free to
correct me) -

> 2) would using a hardware card (such as a 3ware 7006-2 Parallel ATA
> RAID Controller) decrease the amount of time to sync?

If you're using (true) hardware RAID, you don't need to use the mdadm
tools, so the "sync time" is the time it takes your RAID card's BIOS to
construct the array.

I've never used a 3Ware P-ATA card. I have used 3Ware SATA cards; IIRC
creating RAID sets didn't take more than an hour or two.

> 3) will it take this long to synchronize the drives after, say, a
> system crash (ie a hard reboot), or does it only take 25 hours for the
> initial sync ?

It might take as long (but it might not be relevant). Let's say you lost
one drive in your sw RAID1 - if you boot the box with one drive missing,
I'd imagine that you'd be running at (or near) full speed, with your
array running in degraded mode.

If you boot the box with one good drive and a replacement drive, I'd
imagine that your rebuild would happen in the background. Your load
average would probably be higher while the rebuild took place, but
may/may not be enough of an increase to render your box unusable while
this is happening.

-Matt Cuttler