[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]

Re: [Linux-cluster] Starter Cluster / GFS

Nicolas Ross wrote:
The volume will be composed of 7 1TB disk in raid5, so 6 TB.

Be careful with that arrangement. You are right up against the ragged edge
in terms of data safety.

1TB disks a consumer grade SATA disks with non-recoverable error rates of
about 10^-14. That is one non-recoverable error per 11TB.

Now consider what happens when one of your disks fails. You have to read
6TB to reconstruct the failed disk. With error rate of 1 in 11TB, the
chances of another failure occurring in 6TB of reads is about 53%. So the
chances are that during this operation, you are going to have another
failure, and the chances are that your RAID layer will kick the disk out
as faulty - at which point you will find yourself with 2 failed disks in a
RAID5 array and in need of a day or two of downtime to scrub your data to
a fresh array and hope for the best.

RAID5 is ill suited to arrays over 5TB. Using enterprise grade disks will
gain you an improved error rate (10^-15), which makes it good enough - if
you also have regular backups. But enterprise grade disks are much smaller
and much more expensive.

Not to mention that your performance on small writes (smaller than the
stripe width) will be appalling with RAID5 due to the write-read-write
operation required to construct the parity which will reduce your
effective performance to that of a single disk.


The enclosure I will use (and already have) is an activestorage's activeraid
in 16 x 1tb config. (http://www.getactivestorage.com/activeraid.php).

I dealt with them before. All I'm going to say is - disregard any and all performance figures they claim and work out what the performance is likely to be from basic principles. Provided you stick to that and ignore the marketing specmanship, as far as enterprisey storage appliances go, those are reasonably good value for money.

drives are Hitachi model HDE721010SLA33. From what I could find, error rate
is 1 in 10^15.

That makes it less bad than my figures above, but still, be careful.

It will host many, many small files, and some biger files. But the files
that change the most often will mos likely be smaller than the blocsize.

That sounds like a scenario from hell for RAID5 (or RAID6).

What do you suggest to acheive size in the range of 6-7 TB, maybe more ?

RAID10 if you need more performance than that of a single disk, unless your I/O operations are always very big (bigger than the RAID stripe width).

stripe_width = chunk_size * number_of_disks

Smaller disks are good for reducing rebuild times, and more smaller disks will give you better performance. It all depends on the nature of the I/O and the performance you require.

The gfs will not be used for io-intensive tasks, that's where the
standalone volumes comes into play. It'll be used to access many files,
often. Specificly, apache will run from it, with document root, session
store, etc on the gfs.

Performance-wise, GFS should should be OK for that if you are running with
noatime and the operations are all reads. If you end up with write
contention without partitioning the access to directory subtrees on a per
server basis, the performance will fall off a cliff pretty quickly.

Can you explain a little bit more ? I'm not sure I fully understand the
partitioning into directories ?

Make sure that only one node only accesses a particular directory subtree (until it gets failed over, that is). If you have multiple nodes simultaneously writing to the same directory with any regularity you will experience performance issues.


[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]