Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
On Wed, Dec 2, 2009 at 9:51 PM, Christopher G. Stach II c...@ldsys.netwrote: - Grant McWilliams grantmasterfl...@gmail.com wrote: Portability is no different with a RAID controller as long as you've standardized on controllers. For this to be true, it would have to be absolute. Since many people have evidence that it is not true, it's not absolute. Controllers of the same revision and firmware version have had portability problems. -- Christopher G. Stach II Like I said in our environment we have hundreds of drives moving between controllers *every month and have had zero problems with it. Not all machines use the same controller (some are 3ware 9550SX and some are 3ware 9650SE) nor do they use the same firmware version. We've been doing this for 3 years now and have never had a drive have problems. All drives are initialized, partitioned, formatted and populated with content in 3 locations around the world and then multiple sets are shipped to 75 machines in various geographical zones and used for one month. At this point we've done close to 10,000 swaps (300/month) and we've never had a controller not see a drive and recognize it without anything more than a tw_cli rescan. You can talk theoretics but I can tell you my real world experience. I cannot speak for other vendors but for 3ware this DOES work and is working so far with 100% success. I have a bunch of Areca controllers too but the drives are never moved between them so I can say how they'd act in that circumstance. Grant McWilliams ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
On Wed, Dec 2, 2009 at 9:48 PM, Christopher G. Stach II c...@ldsys.netwrote: - Grant McWilliams grantmasterfl...@gmail.com wrote: Interesting thoughts on raid5 although I doubt many would agree. That's okay. We all have our off days... Here's some quality reading: http://blogs.sun.com/bonwick/entry/raid_z http://www.cyberciti.biz/tips/raid5-vs-raid-10-safety-performance.html http://www.miracleas.com/BAARF/RAID5_versus_RAID10.txt http://www.miracleas.com/BAARF/1.Millsap2000.01.03-RAID5.pdf http://www.codinghorror.com/blog/archives/001233.html http://web.ivy.net/carton/rant/ml/raid-raid5writehole-0.html Maybe you are thinking of RAID 6. I don't see how the drive type has ANYTHING to do with the RAID level. IOPS, bit error ratio, bus speed, and spindle speed tend to factor in and are usually governed by the drive type. (The BER is very important for how often you can expect the data elves come out and chew on your data during RAID 5 rebuilds.) You will use those numbers to calculate the number of stripe segments, controllers, and disks. Combine that with the controller's local bus, number of necessary controllers, host bus, budget, and other business requirements and you have a RAID type. a RAID 10 (or 0+1) will never reach the write... performance of a RAID-5. (*cough* If you keep the number of disks constant or the amount of usable space? Things working tends to trump CapEx, despite the associated pain, so I will go with amount of usable space.) No. -- Christopher G. Stach II Nice quality reading. I like theories as much as the next person but I'm wondering if the Toms Hardware guys are on crack or you disapprove of their testing methods. http://www.tomshardware.com/reviews/external-raid-storage,1922-9.html Grant McWilliams Some people, when confronted with a problem, think I know, I'll use Windows. Now they have two problems. ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
- Grant McWilliams grantmasterfl...@gmail.com wrote: On Wed, Dec 2, 2009 at 9:48 PM, Christopher G. Stach II c...@ldsys.net wrote: - Grant McWilliams grantmasterfl...@gmail.com wrote: a RAID 10 (or 0+1) will never reach the write... performance of a RAID-5. (*cough* If you keep the number of disks constant or the amount of usable space? Things working tends to trump CapEx, despite the associated pain, so I will go with amount of usable space.) No. -- Christopher G. Stach II Nice quality reading. I like theories as much as the next person but I'm wondering if the Toms Hardware guys are on crack or you disapprove of their testing methods. http://www.tomshardware.com/reviews/external-raid-storage,1922-9.html They used a constant number of disks to compare two different hardware implementations, not to compare RAID 5 vs. RAID 10. They got the expected ~50% improvement from the extra stripe segment in RAID 5 with a serial access pattern. Unfortunately, that's neither real world use nor the typical way you would fulfill requirements. If you read ahead to the following pages, you have a nice comparison of random access patterns and RAID 10 coming out ahead (with one less stripe segment and a lot less risk): http://www.tomshardware.com/reviews/external-raid-storage,1922-11.html http://www.tomshardware.com/reviews/external-raid-storage,1922-12.html -- Christopher G. Stach II ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
On Thu, Dec 3, 2009 at 6:08 AM, Christopher G. Stach II c...@ldsys.netwrote: - Grant McWilliams grantmasterfl...@gmail.com wrote: On Wed, Dec 2, 2009 at 9:48 PM, Christopher G. Stach II c...@ldsys.net wrote: - Grant McWilliams grantmasterfl...@gmail.com wrote: a RAID 10 (or 0+1) will never reach the write... performance of a RAID-5. (*cough* If you keep the number of disks constant or the amount of usable space? Things working tends to trump CapEx, despite the associated pain, so I will go with amount of usable space.) No. -- Christopher G. Stach II Nice quality reading. I like theories as much as the next person but I'm wondering if the Toms Hardware guys are on crack or you disapprove of their testing methods. http://www.tomshardware.com/reviews/external-raid-storage,1922-9.html They used a constant number of disks to compare two different hardware implementations, not to compare RAID 5 vs. RAID 10. They got the expected ~50% improvement from the extra stripe segment in RAID 5 with a serial access pattern. Unfortunately, that's neither real world use nor the typical way you would fulfill requirements. If you read ahead to the following pages, you have a nice comparison of random access patterns and RAID 10 coming out ahead (with one less stripe segment and a lot less risk): http://www.tomshardware.com/reviews/external-raid-storage,1922-11.html http://www.tomshardware.com/reviews/external-raid-storage,1922-12.html -- Christopher G. Stach II So if I have 6 drives on my RAID controller which do I choose? If I have to add two more drives to the RAID 10 to equal the performance of a RAID 5 I could just make it a RAID 5 and be faster still. RAID 5 is faster than RAID 10 for reads and writes. However, you are right on the IOs. The RAID 10 pretty much trounced RAID 5 on IOs in all tests. What wasn't in the test (but is in others that they've done) is RAID 6. I'm not sure I'm sold on it because it gives us about the same level of redundancy as RAID 10 but with less performance than RAID 5. Theoretically it would get soundly trounced by RAID 10 on IOs and maybe be slower on r/w transfer as well. Grant McWilliams Some people, when confronted with a problem, think I know, I'll use Windows. Now they have two problems. ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
- Grant McWilliams grantmasterfl...@gmail.com wrote: RAID 5 is faster than RAID 10 for reads and writes. *Serial* reads and writes. That is not the access pattern that you will have in most virtualization hosts. What wasn't in the test (but is in others that they've done) is RAID 6. I'm not sure I'm sold on it because it gives us about the same level of redundancy as RAID 10 but with less performance than RAID 5. Theoretically it would get soundly trounced by RAID 10 on IOs and maybe be slower on r/w transfer as well. RAID 6 is pretty slow, but you can stripe them as RAID 60. If you need that kind of fault tolerance, the performance hit is negligible. On high volume boxes with low performance requirements, say NLS on an 8-12 bay 2U or 3U machine, I use RAID 6 with one hot spare. -- Christopher G. Stach II ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
On 12/3/2009 7:35 AM, Grant McWilliams wrote: You can talk theoretics but I can tell you my real world experience. I cannot speak for other vendors but for 3ware this DOES work and is working so far with 100% success. I have a bunch of Areca controllers too but the drives are never moved between them so I can say how they'd act in that circumstance. Brand probably matters a lot. The 3ware and Areca's I'm inclined to trust. They're true hardware RAID controllers and not just fakeraid. Things get a lot murkier when you get into the bottom half of the market. But for smaller shops that can't afford to have 4+ of everything and don't need the CPU offload that a hardware RAID controller offers, Linux Software RAID is a solid choice. ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
Grant McWilliams grantmasterfl...@gmail.com writes: So if I have 6 drives on my RAID controller which do I choose? considering the port-cost of good raid cards, you could probably use md and get 8 or 10 drives for the same money. It's hard to beat more spindles for random access performance over a large dataset. (of course, the power cost of another 2-4 drives is probably greater than that of a raid card) ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
I have had great luck with nvidia fakeraid on RAID1, but I see there are preferences for software raid. I have always heard that fakeraid and software RAID perform the same. Neil -- Neil Aggarwal, (281)846-8957, http://UnmeteredVPS.net CentOS 5.4 VPS with unmetered bandwidth only $25/month! No overage charges, 7 day free trial, Google Checkout accepted ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
On 12/02/2009 06:30 PM, Neil Aggarwal wrote: I have had great luck with nvidia fakeraid on RAID1, but I see there are preferences for software raid. I have always heard that fakeraid and software RAID perform the same. performance wise they are the same 'cause fakeraid is still a software implementation. flexibility and portability wise ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
- Ben M. cen...@rivint.com wrote: I have had great luck with nvidia fakeraid on RAID1, but I see there are preferences for software raid. I have very little hands on with full Linux software RAID and that was about 14 years ago. MD RAID. I'd even opt for MD RAID over a lot of hardware implementations. This writeup summarizes a bit of why: http://jeremy.zawodny.com/blog/archives/008696.html Hardware RAID's performance is obviously going to be better, but it's only worth it if you *need* it (more than ~8 disks, parity). If you're just doing RAID 0, 1, or 10 in a single box and you're not pushing it to its limits as a DB server or benchmarking and going over it with a magnifying glass, you probably won't notice a difference in performance. I'll take fewer moving parts and portability. As someone already said, dmraid is done in software, too. Fakeraid is basically the same as MD RAID, but with an extra piece of hardware and extra logic bits to fail. -- Christopher G. Stach II ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
Thanks. The portability bonus is a big one. Just two other questions I think. - Raid1 entirely in dom0? - Will RE type HDs be bad or good in this circumstance? I buy RE types but have recently become aware of the possibility where TLER (Time-Limited Error Recovery) can be an issue when run outside of a Raid, e.g. alone on desktop machine. I do have a utility where I can change the HDs firmware setting to get turn it off or on for either Read or Write delays. Christopher G. Stach II wrote: - Ben M. cen...@rivint.com wrote: I have had great luck with nvidia fakeraid on RAID1, but I see there are preferences for software raid. I have very little hands on with full Linux software RAID and that was about 14 years ago. MD RAID. I'd even opt for MD RAID over a lot of hardware implementations. This writeup summarizes a bit of why: http://jeremy.zawodny.com/blog/archives/008696.html Hardware RAID's performance is obviously going to be better, but it's only worth it if you *need* it (more than ~8 disks, parity). If you're just doing RAID 0, 1, or 10 in a single box and you're not pushing it to its limits as a DB server or benchmarking and going over it with a magnifying glass, you probably won't notice a difference in performance. I'll take fewer moving parts and portability. As someone already said, dmraid is done in software, too. Fakeraid is basically the same as MD RAID, but with an extra piece of hardware and extra logic bits to fail. ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
On 12/02/2009 11:49 PM, Ben M. wrote: Thanks. The portability bonus is a big one. Just two other questions I think. - Raid1 entirely in dom0? that's what I do, for simplicity sake. I do all raid in Dom0, usually also LVM (sometime I do use simple/plain/old partitions in dom0 and LVM is domU, but very rarely). after that it's fairly easy to create a new LV (or simply a file in /var/lib/xen/images -- even if it's just bind-mounted from another disk) and use it for the new VMs ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
Ben M. cen...@rivint.com writes: Thanks. The portability bonus is a big one. Just two other questions I think. - Raid1 entirely in dom0? that's what I do. I make one big md0 in the dom0, then partition that out with lvm. - Will RE type HDs be bad or good in this circumstance? I buy RE types but have recently become aware of the possibility where TLER (Time-Limited Error Recovery) can be an issue when run outside of a Raid, e.g. alone on desktop machine. generally speaking the re or enterprise drives are all I use in production (all of production is software raid'd.) otherwise, even though you have a mirror, if one drive fails in a certain way (happened to me twice before I switched to 'enterprise' drives) the entire box hangs waiting on that one bad drive. ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
- Ben M. cen...@rivint.com wrote: Thanks. The portability bonus is a big one. Just two other questions I think. - Raid1 entirely in dom0? That's how I do it. dom0 should be handling the supply of the hardware services, and if dom0 has all of the drivers and physical disks under its control, it counts. dom0 should be able to handle the block scheduling better. Now, if booting guests with iSCSI disks and all the LUNs are on a SAN or otherwise networked and not directly under dom0 control, I would handle it differently. Besides, you don't want the added complexity of managing all of the LVs and extra guest configuration. - Will RE type HDs be bad or good in this circumstance? I buy RE types but have recently become aware of the possibility where TLER (Time-Limited Error Recovery) can be an issue when run outside of a Raid, e.g. alone on desktop machine. Definitely good as long as you have more than one disk in it. :) You want your disk to time out instead of holding up the array. You usually can't go wrong with NCQ and TLER. NCQ won't get you much in RAID 1, though. You can turn it off with the driver in most cases. -- Christopher G. Stach II ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
- Grant McWilliams grantmasterfl...@gmail.com wrote: He had a two drive RAID 1 drives and at least one of them failed but he didn't have any notification software set up to let him know that it had failed. And since that's the case he didn't know if both drives had failed or not. I wonder why he things software RAID would be a) more reliable b) fix itself magically without telling him. He never did say if he was able to use the second disk. I have 75 machines with 3ware controllers and on the very rare occasion that a controller fails you plug in another one and boot up. I have a pile of various RAID controllers from 3ware, Promise, LSI, the utter garbage that older PERCs were, etc. that have pissed me off by randomly dropping disks, not rebuilding or detecting its own disks, hanging, etc. and very importantly, not allowing me to move disks between machines. That being said, I still have two 3ware cards and some LSI that are fine, but most of my arrays are software. Anecdotal evidence aside, unless you know what kind of performance you need, you have usage metrics, and you know how to benchmark properly, you probably don't need the risk and marginal performance improvement of some extra hardware. (This was originally about fakeraid, wasn't it?) I don't use software RAID in any sort of production environment unless it's RAID 0 and I don't care about the data at all. I've also tested the speed between Hardware and Software RAID 5 and no matter how many CPUs you throw at it the hardware will win. I don't allow RAID 5, so the increased checksum processing performance doesn't have any bearing on my choices. :) Even in the case when a 3ware RAID controller only has one drive plugged in it will beat a single drive plugged into the motherboard if applications are requesting dissimilar data. One stream from an MD0 RAID 0 will be as fast as one stream from a Hardware RAID 0. Multiple streams of dissimilar data will be much faster on the Hardware RAID controller due to controller caching. True. That probably falls under be the aforementioned need. :) You can't beat the performance of a cache, although the Linux filesystem cache will perform reasonably well. If you're using something like an ACID compliant DB, you'll want the battery backed hardware cache for any sizable amount of I/O. Discussing performance without discussing benchmark methodology is annoying and often useless, but if you want to go down that route... (Also, your use case is contrary to your storage design. You would use RAID 0 for serial access and something parallel for random access. Yes, a Ford Explorer doesn't hug corners at 120 mph.) -- Christopher G. Stach II ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
Thanks for sharing Grant. Your point about hardware raid is well taken. However, the discussion is about Fake-Raid vs. Software RAID1 and controller/chipset dependence and portability. The portability of a software RAID1 hard drive to an entirely different box is, I have learned, much higher and less time consuming. Grant McWilliams wrote: He had a two drive RAID 1 drives and at least one of them failed but he didn't have any notification software set up to let him know that it had failed. And since that's the case he didn't know if both drives had failed or not. I wonder why he things software RAID would be a) more reliable b) fix itself magically without telling him. He never did say if he was able to use the second disk. I have 75 machines with 3ware controllers and on the very rare occasion that a controller fails you plug in another one and boot up. I don't use software RAID in any sort of production environment unless it's RAID 0 and I don't care about the data at all. I've also tested the speed between Hardware and Software RAID 5 and no matter how many CPUs you throw at it the hardware will win. Even in the case when a 3ware RAID controller only has one drive plugged in it will beat a single drive plugged into the motherboard if applications are requesting dissimilar data. One stream from an MD0 RAID 0 will be as fast as one stream from a Hardware RAID 0. Multiple streams of dissimilar data will be much faster on the Hardware RAID controller due to controller caching. Grant McWilliams ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
Grant McWilliams grantmasterfl...@gmail.com writes: I don't use software RAID in any sort of production environment unless it's RAID 0 and I don't care about the data at all. I've also tested the speed between Hardware and Software RAID 5 and no matter how many CPUs you throw at it the hardware will win. Even in the case when a 3ware RAID controller only has one drive plugged in it will beat a single drive plugged into the motherboard if applications are requesting dissimilar data. One stream from an MD0 RAID 0 will be as fast as one stream from a Hardware RAID 0. Multiple streams of dissimilar data will be much faster on the Hardware RAID controller due to controller caching. Personally, I never touch raid5, but then, I'm on sata. I do agree that there are benifits to hardware raid with battery backed cache if you do use raid5 (but I think raid5 is usually a mistake, unless it's all read only, in which case you are better off using main memory for cache. you are trading away small write performance to get space; with disk, space is cheap and performance is expensive, so personally, if I'm going to trade I will trade in the other direction.) However, with mirroring and striped mirrors (I mirror everything; even if I don't care about the data, mirrors save me time that is worth more than the disk.) my bonnie tests showed that md was faster than a $400 pcie 3ware. As far as I can tell, there's not much advantage to hardware raid on SATA; if you want to spend more money, get SAS. The spinning disks are going to be the slowest thing in your storage system by far. battery backed cache is cool for writes, but most raid controllers have such puny caches, it doesn't really help much at all except in the case of small writes to raid5. ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
On Wed, Dec 2, 2009 at 2:44 PM, Ben M. cen...@rivint.com wrote: Thanks for sharing Grant. Your point about hardware raid is well taken. However, the discussion is about Fake-Raid vs. Software RAID1 and controller/chipset dependence and portability. The portability of a software RAID1 hard drive to an entirely different box is, I have learned, much higher and less time consuming. Portability is no different with a RAID controller as long as you've standardized on controllers. Our situation is not standard but hundreds of drives are swapped between machines throughout the month without an issue. We use RAID sets to transfer large amounts of data (TBs) between machines. All systems use 3ware 9550 or 9650 controllers though so you throw the set in and rescan an it's there. Grant McWilliams ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
Personally, I never touch raid5, but then, I'm on sata. I do agree that there are benifits to hardware raid with battery backed cache if you do use raid5 (but I think raid5 is usually a mistake, unless it's all read only, in which case you are better off using main memory for cache. you are trading away small write performance to get space; with disk, space is cheap and performance is expensive, so personally, if I'm going to trade I will trade in the other direction.) Interesting thoughts on raid5 although I doubt many would agree. I don't see how the drive type has ANYTHING to do with the RAID level. There are different RAID levels for different situations I guess but a RAID 10 (or 0+1) will never reach the write or read performancehttp://www.tomshardware.com/reviews/external-raid-storage,1922-9.htmlof a RAID-5. The disk space waste isn't too much of a problem anymore because as you say drives are getting much cheaper. Although on that subject I'll mention that enterprise drives and desktop drives are NOT the same thing. We deal in hundreds of drives and see about a 3% failure on desktop drives and only a fraction of that on enterprise drives. I will say though that in my opinion the one really important thing to consider is the price. These controllers aren't cheap and if you skimp you will pay. For sequential single reads (streaming one stream) I'd consider using a software RAID 0. For a mirror I'd consider Software RAID but once I get serious and go for RAID5 or RAID6 I'd only use Hardware RAID. That's my 2 cents. :-) Grant McWilliams ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
On 12/03/2009 03:08 AM, Grant McWilliams wrote: Personally, I never touch raid5, but then, I'm on sata. I do agree that there are benifits to hardware raid with battery backed cache if you do use raid5 (but I think raid5 is usually a mistake, unless it's all read only, in which case you are better off using main memory for cache. you are trading away small write performance to get space; with disk, space is cheap and performance is expensive, so personally, if I'm going to trade I will trade in the other direction.) Interesting thoughts on raid5 although I doubt many would agree. I don't see how the drive type has ANYTHING to do with the RAID level. There are different RAID levels for different situations I guess but a RAID 10 (or 0+1) will never reach the write or read performance http://www.tomshardware.com/reviews/external-raid-storage,1922-9.html of a RAID-5. The disk space waste isn't too much of a problem anymore because as you say drives are getting much cheaper. Although on that subject I'll mention that enterprise drives and desktop drives are NOT the same thing. We deal in hundreds of drives and see about a 3% failure on desktop drives and only a fraction of that on enterprise drives. I will say though that in my opinion the one really important thing to consider is the price. These controllers aren't cheap and if you skimp you will pay. For sequential single reads (streaming one stream) I'd consider using a software RAID 0. For a mirror I'd consider Software RAID but once I get serious and go for RAID5 or RAID6 I'd only use Hardware RAID. and none of this options is the answer to the problem FakeRaid or Software Raid :)) ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
Grant McWilliams grantmasterfl...@gmail.com writes: Interesting thoughts on raid5 although I doubt many would agree. I don't see how the drive type has ANYTHING to do with the RAID level. raid5 tends to suck on small random writes; SATA sucks on small random anything, so your worst-case (and with my use case, and most 'virtualization' use cases, I you spend almost all your time in the worst-case) is much worse than raid5 on SAS, which has reasonable random performance. The other reason why I think SATA vs SAS matters when thinking about your raid card is that the port-cost on a good raid card is so high that you could almost double your spindles at sata prices, and without battery backed cache, the suckage of RAID5 is magnified, so soft-raid5 is generally a bad idea. There are different RAID levels for different situations I guess but a RAID 10 (or 0+1) will never reach the write or read performancehttp://www.tomshardware.com/reviews/external-raid-storage,1922-9.htmlof a RAID-5. I understand raid5 is great for sequential, but I think my initial OS install is the last sequential write any of my servers ever see. My use-case, you see, is putting 32GiB worth of VPSs on one mirror (or a stripe of two mirrors) It is all very random, just 'cause you have 30+ VMs on a box. If you do lots of sequential stuff, you will see very different results, but the vitalization use case is generally pretty random, because multiple VMs, even if they are each writing or reading sequentially, make for random disk access. (now, why do my customers tolerate this slow sata disk? because it's cheap. Also, I understand my competitors use similar configurations.) ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Slightly OT: FakeRaid or Software Raid
- Grant McWilliams grantmasterfl...@gmail.com wrote: Portability is no different with a RAID controller as long as you've standardized on controllers. For this to be true, it would have to be absolute. Since many people have evidence that it is not true, it's not absolute. Controllers of the same revision and firmware version have had portability problems. -- Christopher G. Stach II ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
