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

On 4/3/06, Matt Cuttler <mcuttler@bnl.gov> wrote:
> >
> > 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.

ah, good suggestions, I didn't even check to make sure the drives are
operating at UDMA 133.

> 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.

okay, this is good to know, since I wasn't sure how much of the speed
decrease was caused by the CPU performing whatever housekeeing tasks
are required to build the initial RAID mirror.

Now, is there any safe way to stop/pause/suspend/resume a
synchronization?  Because I have a feeling that when I get to work
tommorow, the system will still be 6 or 7 hours away from being
synchronized, and I'd like to try adjusting some of these options,
then resume the sync.  I read on another mailing list somewhere that
it wasn't possible to suspend/resume a synchronization, but then this
was an old message, so I'm not sure if it's still applicable.. If the
only option is to stop the sync, I'd still like to know that procedure
if anybody knows.

> 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?).

I have a slave (CD ROM) on the same channel as /dev/hdc - I'll try
disconnecting it tomorrow and running hdparm to see what kind of
throughput I get.

> 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.

This is important, because we're using this machine as an Asterisk PBX
server, and at the moment, while doing the synchronization through
software RAID, the system becomes unusable, ie voice transmission is
choppy and intermittent on phone calls, which is unacceptable for a
production phone system.

> > 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.

again, referring to the fact that the system is unusable during a
synchronization, it seems as though hardware RAID may be the only
option.  Unless, of course, I can decrease the synchronization time to
a reasonable amount (5 or so hours seems fine - it just means I'd have
to wait until night time to run the synchronization)

If I do decide to go with hardware RAID, would it be worth it to
invest in a Serial ATA RAID card, or would a Parallel ATA card
suffice?  Because at the moment the drives we have are 160 gig IDE
drives, so it'd be cheaper if we can just stick with these, rather
than have to buy new SATA drives as well as the controller card.  But
I don't want to limit myself if there are other attractive features of
using SATA RAID that I'm not aware of (hot swappable after drive

Thanks to all those who replied, your suggestions and advice are very
much appreciated!


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