SATA or SCSI drives - Multiple Read/write speeds.

Robin Laing Robin.Laing at
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.
> <sigh>
> | 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 
than swap?

> |   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 
this before.

> | 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.

Robin Laing

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