Loic Dachary a écrit :
Last time I checked (5 years ago ;-) the *effective* transfert
rate between SCSI and IDE was equivalent (modulo 20% which is not
significant for our needs).
You're right. ONE SCSI Drive has about the same speed of ONE IDE Drive
(moduloa some percent). BUT it is not where the benefit is. My english
is not very good so i try to explain with a simple but true example.
imagine we have some IDE drives en SCSI drive that transfer at 20 MB/s.
if you have only one drive (either SCSI or IDE) there won't be any
benefit of SCSI. BUT if you have a lot of SCSI drive (we can have 16 on
one bus) and use RAID card to do RAID0 (stripping : a file will cut in
little part and write on all drive), the benefit will be there. when you
want to read a big file wrote on a RAID0 device (with, for the example,
8 Real drives behind this), you can have 8 x 20 Mb/s (one SCSI BUS
several drive can speak at the same time, instead of IDE where only one
drive can speak at a time) = 120 MB/s. with 16 Drives you can have about
320 MB/s (theoric of course, but you can expect it in very good case :
big file wrote on all drives).
This is what i mean when i said that it is better to have many SCSI
drive, instead of only big one (where we will see no benefit).
THe other part is RAID0 or RAID5 (most secure) is done by the RAID card,
not the main CPU. So the main CPU can do other task (serving file as NFS
server for example).
Do you figure we can expect an effective 100MB/s transfert rate
on SCSI3 disks nowadays ? This is a newbie question, I know ;-)
Not with one drive, but with several drive : YES. more SCSI drive we
have, better the performance will be.
In some case we can do the same thing with very expensive SATA card. but
not tested by me.
Handling a RAID card was (is ?) a pain for little benefit
compared to RAID5 soft (benefit is only that your performances don't
drop while reconstructing a damaged drive). Again, old memories. Would
you recommend a RAID5 card with native linux kernel support and
full featured management software (failure notification + reconstruction
replacement) shipped with Debian ?
it exists some SCSI RAID card that work perfectly with linux (Adaptec,
Compaq SMART Array ...). I worked with Compaq SMART Array without any
problem (easy to configure, full support in linux kernel ..)
If Free Software sysadmin support + drivers are here for a widely
distributed RAID5 card (last thing we want is to buy a little used RAID5
card that will die or fail on us within 24 month), then I'm go for it.
just do a make menuconfig and see what driver are included in Linux
kernel. Again i did not have any problem with Compaq SMART Array. driver
is opensource and included in Linux kernel (not in a patch). I think it
is not the only one. I just give to you my experience with this card.
I'll research hardware matching to get an idea of how much it
costs. It has to be a 1U machine, that's the trouble. Unless we're ready to
replace the 4 shuttles.
the bad thing with 1U server is that we have only 2 or 3 places for
Drives. In 2U HP server we can put 6 Drives. It is better to have a lot
of drives instead of only one or 2.
I know HP sponsorised linuxfr.org .. maybe they will want to sponsorised
gna with one 2U server (DL380 G4 can have 2 processors, with 6 Drives,
with 4 Go of RAM. it is what we used in Easynet France). It is a good
start for NAS (Network Access Storage) server.
if you have other question, telle them (in english or french ;-))
--
BERGAMO Jean-Louis <[EMAIL PROTECTED]> http://www.j1b.org/gpg.txt
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