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!
