W B Hacker wrote: > Graeme Fowler wrote: > >> Hi >> >> Look! I'm responding to the original question, not getting involved in >> an argument. Woo! >> >> On Thu, 2008-03-20 at 19:35 +0000, Tom Brown wrote: >> >>> I wonder what people think for a RAID level for their mail stores? >>> >> It depends entirely on circumstance. That circumstance comprises >> hardware, number of drives, OS, network load, peak required throughput, >> what apps are sharing the array, required space, future growth, whether >> the system is read- or write-heavy, the size of the files, and a >> multitude of other factors (many of which Bill Hacker has already >> mentioned in the spiralling "which is best" thread). >> >> >>> I can only have RAID 1 or RAID 5 and i dont have many users ~500 so what >>> do people think? This will be on a hardware RAID card but the RAID >>> channel will not be dedicated to the mail store ie there will be another >>> RAID disk sharing the channel. >>> > > 'Only RAID 1 or RAID5' hints what is on-hand is not a bottom-end > psuedo-controller (most cannot do RAID5, only 0,1, or 0+1), but neither > is it a high-end (which can also do a few more fallen-from-grace > versions, and the current buzzphrase - RAID 6). > > Leaving SCSI out for the moment, it is now hard to even find in stock > new 3.5" drives under 200 GB or 2.5" under 80 GB. > > But the 'sweet spot' in bang-for-the buck is now around 320 to 500 GB > 3.5", 120 to 160 GB 2.5". > > Even if you go all-IMAP, no POP, 500 typical users are not likely to > fill up a pair, or two sets - of 500 to 750 GB SATA drives fast enough > that you won't have plenty of time to plan as they grow. > > If you really want to reduce risk of failure, buy decent, but not > stupidly expensive drives, do about 30-60 days 'burn-in' on another box > (or as hot standby in the same box), migrate the data and swap-out the > older production drives at not over 2-year intervals or 60% full - > whichever comes first. You *can* even 'leapfrog' intentionally > mismatched RAID1 (say a 500 GB with a 750, then activate another slice > and go 750 with 1 TB) to further shorten the age of the rig, but it is > more work. > > The 'pulls' go into lighter / less critical duty, and you keep on with > hand-me-down - replacing only the drives at the top of the risk chain. > > If you slice/partition intelligently, the natural progression of > larger-cheaper drives make for a cheap and cheerful growth with minimal > admin time, and the opportunity to re-optimize every year or two to cope > with shifting needs. > > And RAID1 = with pure mirrored data, no parity - fits the 'cheap and > cheerful' model quite well, as it is largely portable across controllers > and even back and forth between 'hardware' RAID and software RAID. > > RAID5 is not so easy to move about, and wo betide you if you have a > controller failure, have no identical spare, and cannot find an exact > match 'Real Soon' - or ever. > > YMMV > > Bill > > > >> You may want to refer to some of the RAID resources on the web. Googling >> for "raid levels" is a good start. >> >> In essence, given that you have a choice of RAID1 or RAID5 you're >> choosing between the following (or not; depending on the number of >> spindles you require): >> >> RAID1 >> Mirroring. >> A given pair of disks act as a single drive. >> Writes can be slower than a single disk (all drives in the mirror have >> to report success for a write to complete). >> > > To the underlying OS's VFS layer, ultimately, yes. > > To Exim, not. It has gone away to do other things. > > Not on a Unix anyway. Nor AFAIK on all Linux fs, either. > > >> Reads can be faster than a single disk (reads can come from any disk in >> the mirror; where files exceed a single block size the file can be read >> from multiple disks simultaneously). >> No loss of data with failure of a single disk. >> >> RAID5 >> Stripe + Parity >> The data and parity blocks are striped across all spindles within the >> array. >> Poor write performance - each write takes multiple operations; read/read >> parity/calc parity/write data+parity. A good controller does this in RAM >> (cache) and flushes to disk some time later. A better controller has >> battery backup so in the event of a power failure it can flush the cache >> after power is restored, leaving the data intact. >> No loss of data with failure of a single disk. >> >> > But potential speed hit, especially if it is not distributed parity, and > it was the parity drive that went tits-up. Which - given they do the > most I/O - does happen. > > IOW - a RAID5 with a failed component can slow down even if NOT actively > rebuilding. A RAID1 only suffers a performance hit *while* rebuilding. > > >>> Just want to lay the spindles out 'correctly' >>> >> How many are you intending to have? >> >> In my experience, 500 users is fairly small and will not hit you so >> hard; RAID5 will probably be a good compromise for you since it will >> give you the maximum amount of data for a given set of spindles (above >> 3). RAID1 essentially gives you half of your theoretical capacity; RAID5 >> is harder to calculate but, for example, using a 6-spindle array of >> 100GB disks with no spare drive you'd have in the region of 475GB >> available (this will shrink or grow according to vendor parity >> calculations, "right-sizing", and a number of other factors). >> >> > > Price 6 ~100 GB SCSI (SATA 100 GB are history) AND a decent RAID5 > controller, vs 2 X 750 GB SATA. No special controler needed. > > >> It's also worth remembering that vendors sell disks according to "Giga" >> equalling 1000, not 1024 - so your actual disk size is likely to be >> slightly smaller anyway. As an example, a RAID1 mirror of two 750GB >> vendor-sized disks shows me: >> >> Array Size : 730860992 (697.00 GiB 748.40 GB) >> >> The GiB being the actual Gigabytes in base 16 rather than base 10. >> Anyway, that's a digression. >> >> > > But useful - priced. > > >> As you mention the fact that you only have level 1 or 5 available, how >> many spindles can your hardware controller have attached? >> >> Graeme >> >> > > Figure you need 2 SATA channels per RAID1 and Tyan, Serverworks, > GigaByte often have 6 to 8 onboard = 3 to 4 arrays w/o even an add-on > card or port mux. Boot, the OS, utils, and apps on a pair of 80-160 GB > 2.5", 'read mostly' so spends most time in fast RAM, 2 arrays of 2 X 500 > GB each for queue, logs, and mailstore and you are looking at 1+TB in > short 2U or long 1U. > > Take the savings vs a RAID5 and build a hot-standby twin to the server, > split the load, keep the full user database and structures on both, > readly for near-seamless IP-takover or DNS re-pointing of POP & IMAP as > well as smtp. > > Bill > > >
thanks all for the responses and also for an enlightening thread - I have 6 spindles available to me and i _could_ also do RAID1+0 but the sacrifice on space is not worth it for me - The disks are 10k SCSI (this is a slightly aged GL 380) and its a 'propper' hardware controller. I have been in this game for ~10 years and apart from a SAMBA server i'd personally never use SATA for server grade kit. I like SAS but alas this is a 'home' machine and so SCSI it is. Thanks for the comments i think i'll go for 3 RAID1's for binary, scan and store cheers all! -- ## List details at http://lists.exim.org/mailman/listinfo/exim-users ## Exim details at http://www.exim.org/ ## Please use the Wiki with this list - http://wiki.exim.org/
