[mythtv-users] Is there a distributed filesystem available?

[Allan Stirling]

I run one large RAID 5, so do have some experience here.

Debabrata Banerjee wrote:
> Rebuild a raid 5, lets say 6 hours. 6 hours > just put in a new disk
During which time your system works fine, without any manual 
action required if it's configured right.


> Expand a raid 5, let's say 20 hours to restripe > just put in a new disk.
Yup. Agree with this one completely. And it's even worse 
than that. Last time I looked at this, restriping was error 
prone (ie not actually officially supported) and required 
the md device to be offline.

> FS corruption on a raid5, all your data gone, system doesn't work
I have never seen FS corruption (you're talking about 
metadata here) using any sensible transactional FS (XFS is 
great in this respect)

> 2+ disks dead on a raid5, all your data gone, system doesn't work.
Yes, this is potentially an issue - But not one that's 
unavoidable. You have a hotspare, ready in case of any disk 
failure. This means you waste 2 disks rather than just one.

If it's a temporary disk failure (connector, psu fault, 
etc), it is possible to rewrite the RAID superblock to 
recover the array, since as soon as the array becomes 
invalid, the kernel will no longer write to the data portion 
of the array. Of course, this means that you *need* to store 
your RAID configuration file somewhere apart from on the RAID ;)

There are further issues with RAID5 - It's write speed is 
not stellar. To give an idea of the difference, with the 
same hardware (8 disk software RAID) doing linear writes I 
have seen:

162Mb/sec from RAID5
220Mb/sec from RAID0

However, this is *very* unlikely to impact a home deployment.

Transactional filesystems are also bad performers by default 
on RAID5, since each block of data writes (actually, 
metadata writes) is preceeded by a write to the log.
This doubly hurts a proper SQL server, since the same 
applies there to the data.

Some of these points bring up an important issue, however. 
RAID is not a substitute for backups.

I'm not saying that backing up all of your data should be 
mandatory, but if you have any way at all to back up stuff 
you care about (family photos, etc) then _do it_. And verify 
that the backup can actually be restored. Personally, I'd 
also back up the database and each of the boot/root (not 
data) volumes on each FE / BE.

Anyway, I just thought I'd share. Apologies for the rambling :)

Cheers,

Allan.