> - for my use (web/email/cgi) why should I choise RAID 5 and not RAID 1 ?
+ RAID 5 makes better use of your disks, but you need at least 3 disks.
- RAID 5 puts higher load on your CPU esp. with write accesses
So if you have a slow, single CPU and you don't need much HDD space, go for
RAID1.
If you have a fast CPU or SMP with fast CPUs and you need much HDD space, go
for RAID5.
> - for RAID 5 we need to put 3 HD. what happen when I put off one
HD ? > the sevver is still running ?
If nothing really bad happens, the system will continue to run in degraded
mode. You may even reboot it and it will come up again fully operational in
degraded mode.
You should take measures that you WILL notice the first disk failure by
monitoring /proc/mdstat - or you will maybe think all is ok until the
second disk fails (and you will lose all data in that case)
> what happens to the datas ?
You only lost redundancy data, so all data is still there.
> what happen when I replug the HD again in the server ?
First you should shut down and power off your system, before plugging
something.
Then, after reinserting it and rebooting, NOTHING will happen.
You need to first fdisk the new disk the same way as the defective one and
then do a raidhotadd manually.
Then reconstruction will begin and after some time, system will be in
redundant state again with all drives up.
> the data not saved for the delay the HD was outside ?
No, the system "knows" that the disks you had operational the whole time have
newer data and it will compute redundancy data based on the current data.
So you won't lose the data you had before the disk failure and you won't lose
data that were changed while the array was in degraded mode (while one disk
was "missing").
> - how about the benchmark with RAID 1 and 5 vs SCSI classis ?
Always assuming that CPU power and SCSI bus bandwidth is NOT a limiting
factor:
RAID1, READ: with mutliple parallel reads, you get better values than with
single disk - but this is difficult to benchmark.
RAID1, WRITE: you get about same performance as with single disk
RAID5, READ: you will have better read performance than with single disk. The
more disks you use, the better performance you will have.
RAID5, WRITE: you will have better write performance than with single disk. The
more disks you use, the better performance you will have. But: you will
have quite some CPU load, so a fast CPU is good for performance (and if your
apps also do a high cpu load, better go for a SMP system).
Do not use too many disks in RAID array as this will lead to an increased
disk failure probability.
Also do not use too many disks on a single SCSI bus, as bus performance will
limit RAID performance to some maximum. If you have at least 4 fast HDDs on a
single bus, you should think about if a second bus (or a faster bus, if
your device support that) maybe will give you additional speed
Thomas
