24th February 22:35
RAID setup help
No 3ware cards use decent onboard CPUs (as do Mylex, formerly part of IBM
but a better part than Lexmark became, and so do a few other brands out there).
o SCSI is best for multi-user RAID environments
---- or where you plan on a very large number of drives
---- or where you emphatically need 15.3k-rpm drives re latency
o SATA/ATA are best for single-user RAID environments
---- or where you need lower TCO re lower drive purchase cost
---- or where you need redundancy of that drive array (ie, 2 distributed machines etc)
The reason is relatively simple:
o SATA drive like a WD Raptor 10k-rpm has a *very* high SDTR capability
---- it's seek time (electromechanical latency) is good, but not the world's fastest
o SATA drives however in the present Drive/Card/OS chain lack NCQ/TCQ
---- NCQ/TCQ is where seeks & access are ordered to minimise head movement
---- SDTR means nothing if the heads are seeking all over the place
---- and HD are slow *electromechanical* devices when it comes to head seeking
o SCSI drives conversely often have a lower SDTR than say a WD Raptor 10k-rpm
---- they have very fast track access times - so low electromechanical latency
---- they have SCSI NCQ/TCQ to re-order seeks/accesses - so low electromechanical
o SCSI drives thus perform best in multi-user environments
---- because in multi-user the stress is not SDTR - but in I/O per second (IOPS)
---- if 50 users are accessing a dbase, the peak SDTR means little - seek times do
---- seek times are lower for SCSI drives & lower since SCSI supports NCQ/TCQ
The latest SATA drives do indeed support NCQ/TCQ - but the Card/Drivers do not,
and the O/S does not as yet. You should see a 10-15% improvement when they do,
and another benefit of SCSI is gone (except for 15.3k-rpm drives having a lower
rotational latency than the present 10k-rpm WD Raptor drives of course - at a price).
There are thus 3 "levels" of RAID:
o Cheap-as-Chips ATA/SATA -- s/w RAID or cheap RAID cards with a BIOS
---- Highpoint, some Promise & most onboard RAID fall into the latter category
---- something like a plug-in Intel RAID adapter doesn't (it has an i960 CPU on it)
o High quality & price ATA/SATA/SCSI - true h/w RAID solutions with onboard CPU
---- Promise do one (FastTrak), 3ware do some up to 8/12 devices, Mylex & Adaptec also
---- SCSI ones cost disproportionately more - the Promise/3ware SATA 4-port are good
o External Fibre Channel RAID solutions - despite the price, often use SATA drives!
---- here it comes down to whether server IOPS matters (SCSI) or cost of data-size
---- various i/f's (not just FC) can be used, basically O/S independent external
---- for example, a data-warehousing or near-line storage (pre-tape) uses SATA RAID
---- conversely, a e-commerce transaction server uses SCSI RAID, clustering etc
I tend to prefer RAID-10 if you have 4 drives - it is both Mirror & Striping.
It has less overhead than RAID-5, and offers good reliability as well.
It's unfortunate 3ware keep their 4-port card ~3x the price of their 2-port version,
although Promise do a good 4-port card. If you RAID a lot of drives, you need to
think about the bottleneck of PCI - more an issue if doing RAID for SDTR. Most
servers actually don't get particularly high SDTR from their SCSI disks since they
are in a multi-user environment, so access-latency eats into the peak figures. That
is particularly true when loading say user-profiles or handling MS-IE temp files etc.
So RAID for a MPEG production server would be 3ware & WD Raptor x4 - since that
is stressing SDTR of a single-user large-dataset, RAID for a major dynamic-dbase
server would go with a SCSI solution since that stresses lots of file accessing/updating
and as much memory as you can stuff in the thing (where Opteron 64-bit eats Xeon).
RAID is not a backup device - it's about availability:
o Cheap "h/w" RAID cards do *not* offer automatic rebuild on drive-replacement
---- you *must* know how to rebuild that array before it happens
---- it can be quite involved and a mistake could cost you data
o Real "h/w" RAID cards do offer automatic rebuild on drive-replacement
---- be it SCSI or ATA/SATA, from 3ware to Adaptec - but at a higher price
---- note some require the RAID config to be stored on a disk - don't lose it :-)
Hard-drives are electromechanical components and thus can suffer failure from such,
and also from heat - often overlooked re case ambient temps or S.M.A.R.T. drive diags.
Windows can RAID disks - but only dynamic disks (which the system disk can not be :-)
If it's just for reliability on a SOHO system, I'd perhaps choose a mid-way option:
o Non-RAID the system disk
o RAID two data-disks with a 3-ware 2-port card (120$/£100) - RAID-1 mirror
---- auto-rebuild, reliable proven card
If it for reliability with performance, I'd choose a higher-end but cheap option:
o RAID-10 all 4 disks (eg, 2 system & 2 data)
o Use a Promise 4-port RAID card as a good bit cheaper than a 3ware option
If it's a commercial environment then 3ware is a good solution - but expensive >2-port.
The cheap RAID cards have a long google history of "help!" if you search around.
http://www.dorothybradbury.co.uk for Panaflo fans (free shipping)