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SCSI vs IDE (was:Re: [vox-tech] Xen + LVM usage)
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SCSI vs IDE (was:Re: [vox-tech] Xen + LVM usage)





SCSI, painful? these massive fileservers you use, what is the file-acces concurrency on them?

I've got around 20 virtual servers (some under moderate load) on one server w/ 10 18G 10K fibre drives. I've got another 40 on 6 10K u160 SCSI drives. before I had customers, I had put 10 virtual servers on a SATA disk system. even though the SATA system was running trivial loads (http, dns, spamfiltering and email for my personal stuff) the sata system would have periods of unresponsiveness, when you'd have to wait 10 seconds or more to open a 5K file with PINE. On the SCSI system, I can count on opening 1GB mail folders in the same 10 seconds. That's what I need... predictable. It doesn't have to be blazingly fast, but it does need to degrade gracefully when overloaded. IDE does not seem to do this.

Modern metadata ordering /caching fileystems (logging or softupdates or whatever) all but eliminate the IDE penalty on write, but on read, you still hit concurrency issues if you have to many users hitting the same IDE disk.

Now, if you only have a small number of concurrent accesses to the filesystem, I agree, IDE is the better choice, simply because it is so cheap and so big. Right now, I'm looking at used SATA -> fibre channel cases on E-bay, with the intent of renting customers IDE disks one at a time. One business model I am considering is to use customer-owned disks; perhaps requiring that they buy the disk from me + pay a up front deposit that is enough to cover return shipping and handling, then just charge $15/month or so for an IDE slot in my SAN. I can then connect the IDE disk to the virtual server the customer requests. If they buy the disk from me, I could even include a "I'll replace it within X hours of when it dies, then handle the RMA myself at no charge" service, as that would be easy for me to do. I just keep a couple extra disks around and swap them as needed; then RMA all the disks once a month. When they quit, I mail the disk back to them, or buy it back for some pre-arranged (time-based) fee. This lowers my initial capital costs, and makes the monthly cost of managed remote disk much more competitive with the costs of throwing your own server full of ide up somewhere else.

Of course, I'm low on both capital and time, so who knows when or if I will implement it... but my point is that I'm not a total IDE bigot.


Then we have the next big thing Serial attached SCSI- from what I understand, it looks an auful lot like those raptor 10K drives. the interesting part here is that there are/will be SAN-style disk aggrigation technology, and SAS is plug-compatable with SATA, so if I get a SAS san up, I will be able to swap in SATA disks using the same attachment technology.

So far the only SAS/SATA aggrigation tech I've seen in the field is the 3ware multi-lane cable that aggrigates 4 SATA cables into one semi-propritary cable (adaptec has something very similar, based on the same standard, but the cables are subtly incompatable. One of my customers got the wrong one; I tried.) and really, it's nothing to get excited about. Fibre channel is a mature, stable and inexpensive technology in comparison. (that is, if you buy used 1G fibre.)



On Mon, 7 Aug 2006, Bill Broadley wrote:
Ugh, SCSI seems expensive and painful these days.  I'm perfectly happy
with, er, 8 or so large (4-6TB) fileservers I run.  I definitely recommend
the enterprise/RAID level SATA drives (usually an extra $10 each).  In any case
that is probably best saved for another thread.

Yes, well, I thought it was an interesting thread.
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