[Nottingham] LVM and load balancing and ReiserFS

[Andy Smith]

Hi Martin,

On Sat, Oct 27, 2007 at 01:43:45PM +0100, Martin wrote:
> I've great prejudice against RAID for the increased latency and the mess
> if it goes wrong. If you're wanting a warm fuzzy feeling of resilience
> for your data, I much prefer using rsync/mirroring onto an independent
> system backed up with good backups all round.

Increased latency?  Software RAID generally outperforms a single
disk and I would certainly prefer to avoid spending the time
rebuilding a machine that loses a single disk.  RAID is not backups
so no point comparing them; you need backups, you may also want
RAID.

> So... I have a large (1 TB +) ever expanding chunk of frequently
> accessed data that can't easily be divided up.

Are you aware that while disk capacities have increased dramatically
over the last few years, the error rate per bit of storage has
stayed the same?  Therefore today's hard drives are actually
objectively less reliable than before..

> So, being as it's got to go across multiple disks, I'd like to have some
> flexibility to swap in/out disks as the data grows further, and also
> gain some IO performance. All without needing to completely rewrite the
> whole lot onto new disks for each upgrade.

You can do this with LVM alone.  You would add all three disks as
physical volumes in LVM, tell LVm to stripe, then if you needed to
remove a disk you would tell LVM to reallocate everything off the
disk you are about to remove onto the others.  Then you would remove
that drive from the volume group, and could then physically remove it.

However I wouldn't recommend it as you'd have no redundancy against
single disk failure.

> One option might be LVM with linear addressing *IF* ReiserFS does (or
> can be configured to) spread the data evenly-ish across the full
> physical storage space.
> 
> Does ReiserFS optimise to use HDD cylinders to minimise head movements?
> If so, will ReiserFS still pick that up from LVM?...

Can't comment on reiserfs.. have never used it and I wouldn't start
using it today given that its future is in doubt.

> Or is LVM clever about striping if you then add another one or two HDDs?

You can tell it to stripe or do it linearly, I can't imagine how
much cleverer you would want it to be, can you explain?  But don't
do it!

> Or any better ideas?

As others have said, do LVM on software RAID, then you should be
able to add and remove disks.

Linux MD driver supports growing most setups except RAID-10 I think
so it's future-proof.  As in if you have 3x500G disks now, you could
replace them by 3x1T disks later and grow the RAID-5 to use all the
extra space while it is online.

If you only wanted to upgrade hardware piecemeal then you could
later add 2x1T configured as another RAID-1 and add that to the LVM
volume group.  Only growing the LVM by adding redundant RAID devices
gives you easily expandable redundant storage.

Cheers,
Andy

-- 
http://bitfolk.com/ -- No-nonsense VPS hosting
Encrypted mail welcome - keyid 0x604DE5DB
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 189 bytes
Desc: Digital signature
Url : http://mailman.lug.org.uk/pipermail/nottingham/attachments/20071027/b7390e8d/attachment.bin