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DigeratiPrime

RAID 1 faster concurrent reads than RAID 0?

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I think RAID 1 is faster then RAID 0 when reading two different files since it can read the seperate files simultanesouly from each disk.

Whrereas RAID 0 would read a part of each file (stripe) from each disk before seeking to the other file and back.

It's just a theory but I am wondering if anyone knows the facts here.

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RAID 0 is faster IO speeds, RAID 1 has faster read speeds than a single disk, but don't knwo how that comapres to read speeds of Raid 0.

Raid 0 is suppose to be fasterm other wise what would be the point? As it has no fault tolerance, what would the benefit be? It can depend ont he actual setup, how many drives you have and how many controllers etc.

http://www.m-techlaptops.com/raid.htm

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To me, RAID0 ceased to be a viable option once RAID5 and RAID10 came out.

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To me, RAID0 ceased to be a viable option once RAID5 and RAID10 came out.

Not really.

RAID5 and RAID10 aren't useful for 2 drive setups (which does bring a very significant speed boost -- beyond that you have diminishing returns anyways). You need 4 or more HDs (all the same) to use 'em which most people don't have (yes, you can do 5 with 3 disks, but it's not much better than 10 still).

Also, RAID10 wastes half of your space (need to buy twice as many drives, doubling costs instantly), and while RAID5 isn't as wasteful in terms of disk space, it eats a lot of CPU power (slows down your computer), unless you bought a very expensive "hardware" RAID controller (the kind with a beefy processor doing the XOR'ing onboard -- hundreds of $$$ for something nice). RAID5 isn't completely safe either (RAID6 is safer, but again, more wasteful)

I use RAID0 for my OS & apps, various places where I do a lot of work with large files (downloading, editing ISOs, AV editing, decompressing stuff, whatever... I wouldn't really care if I lost those things to a dead HD), and RAID1 along with proper backups (of course) for the stuff that matters the most (like pics of the kids).

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Whrereas RAID 0 would read a part of each file (stripe) from each disk before seeking to the other file and back.

I believe this is incorrect. You will seek all parts of the file from all disks at the same time with RAID 0, the more disks you have the faster this operation will perform.

RAID 1 controllers these days support 'split seek' which allows the same function, so again the more disks the faster the operation, though you still take the 2:1 write penalty.

Ive been wronger before :)

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Also, RAID10 wastes half of your space (need to buy twice as many drives, doubling costs instantly), and while RAID5 isn't as wasteful in terms of disk space, it eats a lot of CPU power (slows down your computer), unless you bought a very expensive "hardware" RAID controller (the kind with a beefy processor doing the XOR'ing onboard -- hundreds of $$$ for something nice). RAID5 isn't completely safe either (RAID6 is safer, but again, more wasteful)
Yeah, but how expensive are those disks, really? So it's 16¢ a GB vs 8¢... I guess if you're on a tighter budget two drives are definitely a cheaper build than 4, but if you can afford 4, it makes more sense to do that than say RAID6 or RAID5 with a hot spare, or even RAID0 over 4 drives (at least the 10 has some redundancy).

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Of course if one has the budget for four drives, why not?

But for most people, a pair of say, WD RE3 WD1002FBYS (1TB) is already pricey enough (~$320), 4 is a bit extreme (more like ~$640). Even on cheaper drives, it's the difference between $200 and $400 for disks alone. And it's not so much the total cost, as much as having to buy them all at once (a $100 HD a month doesn't seem too bad, but dropping $400 on 4 drives at once... takes a sizable part of disposable income out of a pay)

4 drives isn't that impressive on the average desktop in RAID0 compared to 2 drives in RAID0 (diminishing returns beyond 2 drives mostly, especially on "cheapo RAID" -- good SSDs are perhaps a better option in a way for perf beyond 2 drives)

4+ drives would be nice for RAID5, if only I could afford a nice Areca card with enough ports (many hundreds of $), ditto for RAID6

4 drives would be nice for RAID10, only if I could afford a bunch of Norco RPC-4020 cases and several RAID cards to fit all my drives in (I got like 16 drives already and I'm VERY low on space, no idea where I'd put 20+ more!)

It's the hundreds more in drive space, it's the hundreds more in fancy RAID cards, it's the hundreds more in big fancy cases to hold all the drives in (and good PSUs and everything else)...

I only wish I could afford that kind of stuff

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Something to shoot for :P

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I use RAID0 for my OS & apps, various places where I do a lot of work with large files (downloading, editing ISOs, AV editing, decompressing stuff, whatever... I wouldn't really care if I lost those things to a dead HD), and RAID1 along with proper backups (of course) for the stuff that matters the most (like pics of the kids).

That sounds like a good way to go about it for people with multiple HDD's that don't want a PC resembling a file server with more than 4 drives :P Stuff that needs to go fast but does not need backups on two RAID 0 configured drives, and stuff that is important on two RAID 1 configured drives.

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Raid-0 does read separate files from different disks when these files are smaller than the stripe size (simplified explanation). This is the normal behaviour of any Raid-0. Some controllers do it in Raid-1 as well, but not nearly all do. And when writing, Raid-1 is worse of course; writing happens every time you read a folder in Windows, unless you lit NtfsDontUpdateLastAccess (name a bit imprecise) in the register.

Stripe size is meant just for the purpose you describe. Having stripes bigger than sectors means that small files probably fit into on single stripe, thus allowing the Raid controller to ask a single disk access the data and use the other(s) disk(s) work on other small files in parallel. For bigger files that span several stripes, all disks work on the same request.

Though, the Raid controller doesn't know about files, alas - the OS only gives it sector numbers. So files sometimes span stripe boundaries though it wouldn't be necessary and this slows the Raid down.

On an OS and applications that would issue only one request at a time, here's how stripe size can be optimized:

- If a request needs to access two disks, latency growth from: arm move +half-turn of a platter turn, to: arm move +two-thirds of a platter turn (because platters aren't synchronized nowadays, because of adaptive zoning) so you first loose 1/6 of a turn, for instance 1.39ms at 7200rpm;

- But then the file is read with twice the throughput, say 200MB/s instead of 100MB/s with two disks;

- With such figures, the breakeven is at 278kB, which is bigger than the average file size of W2k-Xp.

With faster platters (15000rpm scsi, Flash...) stripes can be smaller again. And if all your files are big (pictures, video...) you can choose any stripe size.

However, Nt4 and successors allow software to launch more requests before a pending one is fulfilled. With several requests pending, disks working independently are better. With Ncq as well. This favours bigger stripes.

Beginning with Xp, the prefetch does what applications should have done since Nt4: launch parallel requests. This explains why applications launch faster on Xp than they do on W2k, provided Ram is big enough. With such parallel requests, bigger stripes are better.

One limit is that you want your parallel requests fall on different disks, and if the prefetch calls neighbour files composing an application, they should better be in different stripes.

On a file server, several users and applications issue many requests at any time, so bigger stripes are better.

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To me, RAID0 ceased to be a viable option once RAID5 and RAID10 came out.

Also, RAID10 wastes half of your space (need to buy twice as many drives, doubling costs instantly), and while RAID5 isn't as wasteful in terms of disk space, it eats a lot of CPU power (slows down your computer), unless you bought a very expensive "hardware" RAID controller (the kind with a beefy processor doing the XOR'ing onboard -- hundreds of $$$ for something nice).

Yeah, I'm a 3ware or nothing kinda guy.

:sneaky:

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Raid-0 does read separate files from different disks when these files are smaller than the stripe size (simplified explanation). This is the normal behaviour of any Raid-0. Some controllers do it in Raid-1 as well, but not nearly all do. And when writing, Raid-1 is worse of course; writing happens every time you read a folder in Windows, unless you lit NtfsDontUpdateLastAccess (name a bit imprecise) in the register.

Stripe size is meant just for the purpose you describe. Having stripes bigger than sectors means that small files probably fit into on single stripe, thus allowing the Raid controller to ask a single disk access the data and use the other(s) disk(s) work on other small files in parallel. For bigger files that span several stripes, all disks work on the same request.

This is where I think RAID 1 might be better than RAID 0, reading multiple large files. For example two different DVD video image files (iso) being read simultaneously. With RAID 1 the reading can be done independently by each disk, whereas with RAID 0 the stripes for each file must be read alternating between disks. In this scenario imagine there is two remote users watching video, so latency is important.

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I use RAID0 for my OS & apps, various places where I do a lot of work with large files (downloading, editing ISOs, AV editing, decompressing stuff, whatever... I wouldn't really care if I lost those things to a dead HD), and RAID1 along with proper backups (of course) for the stuff that matters the most (like pics of the kids).

Similar to what I have setup, works the best for me.

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