[linux-lvm] LVM limits?

[Chris Cox]

On Tue, 2008-01-29 at 01:38 +0200, Ehud Karni wrote:
> On Mon, 28 Jan 2008 14:52:20 Joseph L. Casale wrote:
> >
> > >It took less then 90 minutes.
> >
> > 90 Minutes, how "enterprisable" is that? Just cause you can, doesn't
> > mean you should: What if that server tanked during working hours at
> > the corp? You would have to sit and wait for it to come back up.
> > Worse than that are the rebuild times for degraded arrays. Try to
> > rebuild a **huge** array on **huge** discs during production with
> > lots of IO and hope you don't lose another disc. There are practical
> > alternatives with thoughtful management to circumvent the need for
> > and 8EB or larger array, but YMMV :)
> 
> First, that's on my home machine (the whole machine, including the
> disks and controller, cost me about $1100).
> 
> At work I have 1.75 TB VG (RAID-5 using 3ware, 320 GB SATA-2 x 7, with
> 1 spare), but it is split into 3 LVs the largest of them is 700 GB.
> I had several rebuilds, It takes about 14 hours with no disturbance
> at all to the work.
> 
> The fsck problem can be dealt in various ways. Changing the FS may be
> one of them but I have no experience in this way. Another, more ready
> way is to tunefs the FSs (all ext3 in my case) so it will never occurs
> automatically, and do scheduled fsck once or twice a year.

Fine... let's forget fsck (brick wall... beating head... etc.).

Let's say you want to make a copy of your 5TB filesystem... how long
does that take?

My point (washed away in silly talk) is that operations on large
filesytems can take a VERY long time.  Just looking at the (very)
trivial examples and not looking that the problem at a whole doesn't
solve the problem (as much as we'd like to think that it does).

> 
> I think that when we'll have much larger arrays, their speed will also
> be larger. For those who interested in ancient history, I recall that
> 12-13 years ago, I had a Clariion at work with just 40 GB (it was 10
> scsi disks of 4GB) and the fsck (on Aviion DGIX) took more then 50
> minutes. Morale - the advancement of technology work for us.

No.  The performance of arrays is not keeping in step with their
growth.  Not even close.  Granted read/write laser holographic
storage may be a solution, but I doubt we'll see anything in
the traditional "disk" storage medium performance that will catch
us up with regards to the amount of space increases.  It's going
to take something fairly radical.

Even distributed filesystems may not be the scalable answer
for this.... though it might help for a little while.