SATA or SCSI drives - Multiple Read/write speeds.
Robin.Laing at drdc-rddc.gc.ca
Tue Dec 9 11:22:53 EST 2003
Mark Hahn wrote:
> | read the reviews and the hype about SATA being better than IDE/ATA and
> | almost as good as SCSI, even better in a couple of areas.
> | I have talked to our computer people but they don't have enough
> | experience with SATA drives to give me a straight answer.
> there's not THAT much to know.
> | My concern is regarding multiple disk read/writes. With IDE, you can
> | wait for what seems like hours while data is being read off of the HD.
> nah. it's basically just a design mistake to put two active PATA disks
> on the same channel. it's fine if one is usually idle (say, cdrom or
> perhaps a disk containing old archives). most people just avoid putting
> two disks on a channel at all, since channels are almost free, and you
> get to ignore jumpers.
So it would be a good idea to put data and /tmp on a different channel
> | I want to know if the problem is still as bad with SATA as the
> | original ATA drives? Will the onboard RAID speed up access?
> there was no problem with "original" disks. and raid works fine, up until
> you saturate your PCI bus...
> | I know that throughput on large files is close and is usually related
> | to platter speed. I am also pleased that the buffers is now 8mb on
> | all the drives I am looking at.
> one of the reasons that TCQ is not a huge win is that the kernel's cache
> is ~500x bigger than the disk's. however, it's true that bigger ondisk cache
> lets the drive better optimize delayed writes within a cylinder. for non-TCQ
> ATA to be competitive when writing, it's common to enable write-behind
> caching. this can cause data loss or corruption if you crash at exactly the
> right time (paranoids take note).
I forgot about the "write-behind" problem. I have been burned with
> | Main issue is writing and reading swap on those really large files and
> | how it affects other work.
> swap thrashing is a non-fatal error that should be fixed,
> not band-aided by gold-plated hardware.
I agree but I am not looking at swap thrashing in the sense of many
small files. I am looking at 1 or 2 large files that are bigger than
memeory while working. I know on my present workstation I will work
with a file that is 2X the memory and I find that the machine stutters
(locks for a few seconds) every time there is any disk ascess. I
would like to add more ram but that is impossible as there are only
two slots and they are full. Management won't provide the funds.
> finally, I should mention that Jeff Garzik is doing a series of good new SATA
> drivers (deliberately ignoring the accumulated kruft in the kernel's PATA
> code). they plug into the kernel's SCSI interface, purely to take advantage
> of support for queueing and hotplug, I think.
This is interesting. I like the idea of hot-swap drives and this is
one thing I was looking at with SCSI. From this I take it that SATA
can handle some queueing but it just isn't supported yet?
> regards, mark hahn.
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