On Wed, 7 Apr 2004, [ISO-8859-1] G�tz Hoffart wrote:
pertinent,
because just about any problem that can be solved with
faster drives can be solved better with more ram?
In theory: yes. But if more RAM or faster UPM are better should be
tested, this belongs to the kind of application.
Actually, NO. This is a DATABASE SERVER. Databases flush data to disk to
ensure data integrity. No ammount of RAM can speed up the process of an
fsync() as the data must physically be moved from system memory to the
physical drives.
Yes. RAID 10 gives you 50% of the hard disc capacity
while RAID 5 gives
you 66%. That sounds not too much difference but it is :-) 8 drives of
150 GB -> 1200 GB gross capacity. RAID 10: 600 GB net capacity. RAID 5:
about 792 GB net capacity. That's more than one extra hard drive for
"free".
Your math is off... RAID 10 == 50% always. RAID 5 == N-1 (minus any spares)
So, for 8 drives, at best, that's 8-1 == 7 usable drives which is just over
1TB. With a spare, that drops to 900GB.
RAID 10 is a bit faster than RAID 5.
For pure reads, mirroring is better. But in write performance, mirroring
sucks big time. RAID 5 is somewhere in between... it takes less drive
space in trade for a marginal cost in speed. (reads are at single drive
speeds, writes require a block read and two block writes for every block
written -- read the parity block, calculate, and the store the data and
parity blocks)
I would recommend two or three spare hard drives, not
only one.
...
This is merely a waste of drives. If a second drive fails before the
recovery is complete, the array will fail. A RAID 5 array can only
function with a SINGLE drive failure ("degraded".) Until recovery is
complete, the array is in a degraded state.
Multiple drive failure recovery is part of RAID 6 (n-m + spares) which
is still experimental in linux -- and I don't think anyone has tried
it with more than 2 parity strips.
SATA-RAIDs are as fast as SCSI-RAIDs concerning the
sustained read
benchmarks but not when accessing random data - this is what I measured
for our firm here.
No they aren't. At least not the one's generally available (read: the
cheap crap found at retail outlets.) People often forget about Fibre
Channel. I have 5 year old FC (FC-1 1G) drives that hands down beat
the fastest, modern IDE and SATA drives. (Cost-wise (new), they aren't
worth it.) Plus, FC can expand to include >100 drives without any further
hardware (aside from the drives and shelves, of course.)
Again, I'll add my strong recommendation for 3Ware raid contoller cards
(esp. if you're using IDE or SATA drives.)
--Ricky
PS: I still have the bonnie++ results from my testing of raid5 + various
filesystems on my "junk" -- a qla2100, a eurologic FC7 FC shelf, and
7 10k RPM drives (seagate ST118202FC) If anyone is interested, I
can post the numbers.