Hey Guys, I have some questions?regarding?a new home server I am going to build in the hopefully very near future (ASAP, I just need to finish planning everything and this is the penultimate?hurdle), I will be creating a software RAID... Lets say I have three drives "knocking" around which are all 1TB SATA II drives but each made by a different manufacturer. I am going to guess that these couldn't be used in a RAID 5? Or could they? However could a similar result of 2TBs of data with redundancy be achieved with JBOD? Also regarding RAID 5, three drives of data to one for parity is the max ratio I?believe? I.e. to expand this by adding another data drive, the original parity drive would no longer cover this and another would be required, is this correct? One more question about hot swappable drives, I understand that you can create RAID arrays with and without hot swappable drives but I am confused by this concept. I'm my experience with RAIDs I have only every delt with a RAID 1 that has degraded. I simply set the drive as offline, replaced it, set it to online and the RAID rebuilt itself all without restart the server and operation was never interrupted. So we can presume the server had hot swappable drives enabled yes? (It was a hardware RAID). With a software RAID is this still achievable? Thank you for reading. Regards, James ;) Charles de Gaulle ?- "The better I get to know men, the more I find myself loving dogs."
> Lets say I have three drives "knocking" around which are all 1TB SATA > II drives but each made by a different manufacturer. I am going to > guess that these couldn't be used in a RAID 5? Or could they?They can in fact. There might be minor differences of a few sectors between the drives but md RAID will account for those by using the 'smallest' (lowest sector count) drive as the base.> However could a similar result of 2TBs of data with redundancy be > achieved with JBOD?JBOD (Just a Bunch Of Disks) has no redundancy. The redundancy enters when you assign those disks to RAID sets.> Also regarding RAID 5, three drives of data to one for parity is the > max ratio I?believe? I.e. to expand this by adding another data drive, > the original parity drive would no longer cover this and another would > be required, is this correct?No. The parity will be the same size regardless of how many drives are used.> One more question about hot swappable drives, I understand that you > can create RAID arrays with and without hot swappable drives but I am > confused by this concept. I'm my experience with RAIDs I have only > every delt with a RAID 1 that has degraded. I simply set the drive as > offline, replaced it, set it to online and the RAID rebuilt itself all > without restart the server and operation was never interrupted. So we > can presume the server had hot swappable drives enabled yes? (It was a > hardware RAID). With a software RAID is this still achievable?All hot swappable drives allow you to do is replace them without having to completely shutdown the machine. In hardware raid this is often built in such that you can replace a drive without telling RAID ahead of time and it will compensate. IBM xSeries servers are a good example. Software RAID can also do this but you have to tell the RAID system that you plan to remove the drive and then tell it when you add a new drive back. Hot swapping disks is also dependent upon the drive controller supporting hotswap. -- Drew "Nothing in life is to be feared. It is only to be understood." --Marie Curie
James Bensley wrote:> Lets say I have three drives "knocking" around which are all 1TB SATA > II drives but each made by a different manufacturer. I am going to > guess that these couldn't be used in a RAID 5? Or could they?RAID is a manufacturer independent concept. Though depending on who you ask it is typically good to have the same drive and model# in the array, mainly so they have very similar if not identical performance characteristics, if you have some drives that are faster than others then performance won't be consistent.> However could a similar result of 2TBs of data with redundancy be > achieved with JBOD?There's a couple of ways of interpreting JBOD, in my experience the most common way is referring to JBOD as a shelf of dumb disks, often times fiber attached, here is an example of such a system - http://www.infortrend.com/main/2_product/es_f16f-r2j2_s2j2.asp Another way of interpreting it is presenting a bunch of disks without any sort of RAID protection to the OS, either individually or in a concatenated group(set by the host controller).> Also regarding RAID 5, three drives of data to one for parity is the > max ratio I?believe? I.e. to expand this by adding another data drive, > the original parity drive would no longer cover this and another would > be required, is this correct?Depends on the implementation, I can't speak for linux software RAID but it is not too uncommon to have 5 data, 1 parity(5+1), or 8+1, and some even go as high as 12+1 or even higher(shudder). The higher the ratio generally the lower the performance especially on writes, and disk rebuilds will take far longer with bigger ratios, resulting in a better chance of a double disk failure during the rebuild.> hardware RAID). With a software RAID is this still achievable?If the hardware supports it yes. Some controllers don't support hot swap well, especially older ones, and if you yank a drive while the system is running it could crash the system/reboot the box/hang the I/O. But it certainly is possible, just be sure to test it out before putting it into production. If it was me I would go for a 3Ware RAID card, and do it right, only time I might use software RAID these days is if it is RAID 0, which I haven't done since probably 2001. Was considering it for some new web servers because it doesn't matter if a disk dies if we lose the whole box, performance was the most important. But we ended up going with hardware raid 1+0 anyways. nate
James Bensley wrote:> > I have some questions regarding a new home server I am going to build > in the hopefully very near future (ASAP, I just need to finish > planning everything and this is the penultimate hurdle), I will be > creating a software RAID... > > Lets say I have three drives "knocking" around which are all 1TB SATA > II drives but each made by a different manufacturer. I am going to > guess that these couldn't be used in a RAID 5? Or could they? > > However could a similar result of 2TBs of data with redundancy be > achieved with JBOD?If you use software raid to combine the JBOD's, yes.> Also regarding RAID 5, three drives of data to one for parity is the > max ratio I believe? I.e. to expand this by adding another data drive, > the original parity drive would no longer cover this and another would > be required, is this correct?Yes, but if I were doing it I'd either run 2 drives in RAID1 or get another drive and either have 2 RAID1 mount points or a RAID 0+1. The advantage of RAID1 is that you can recover the data from any single disk and it still runs full speed even with a missing disk. RAID5 works but there is a performance hit and a big one when a disk is bad.> One more question about hot swappable drives, I understand that you > can create RAID arrays with and without hot swappable drives but I am > confused by this concept. I'm my experience with RAIDs I have only > every delt with a RAID 1 that has degraded. I simply set the drive as > offline, replaced it, set it to online and the RAID rebuilt itself all > without restart the server and operation was never interrupted. So we > can presume the server had hot swappable drives enabled yes? (It was a > hardware RAID). With a software RAID is this still achievable?Sata drives all but a few controllers are designed to be hot swappable but you need a special drive bay that permits swapping. It probably doesn't matter for a home server where you can shut it down for repair anyway. With software raid, after the drive is recognized (either hotswap or reboot) you need to fdisk a matching partition and then use an 'mdadm --add ...' command to sync a new drive into the raid. -- Les Mikesell lesmikesell at gmail.com
At Tue, 10 Nov 2009 21:26:37 +0000 CentOS mailing list <centos at centos.org> wrote:> > Hey Guys, > > I have some questions?regarding?a new home server I am going to build > in the hopefully very near future (ASAP, I just need to finish > planning everything and this is the penultimate?hurdle), I will be > creating a software RAID... > > Lets say I have three drives "knocking" around which are all 1TB SATA > II drives but each made by a different manufacturer. I am going to > guess that these couldn't be used in a RAID 5? Or could they?They probably could be. The RAID system would use the size of the 'smallest' disk as the base size for each disk. That is if you drives were *actually* 1.02TB, .985TB and 1.12TB, the RAID system would use .985TB of each disk, fully utilizing the .985TB disk, and leaving a 'small' amount of unused space on each of the 1.02TB and 1.12TB. Oh, the sizes will be much closer -- that is you might be lossing only a few sectors here and there.> > However could a similar result of 2TBs of data with redundancy be > achieved with JBOD? > > Also regarding RAID 5, three drives of data to one for parity is the > max ratio I?believe? I.e. to expand this by adding another data drive, > the original parity drive would no longer cover this and another would > be required, is this correct?No. You can use as many disks as you like for RAID 5. The 'parity' is not actually 1 bit. The capacity of a N disk RAID 5 (where N >= 3), is (N-1)*sizeof(one disk).> > One more question about hot swappable drives, I understand that you > can create RAID arrays with and without hot swappable drives but I am > confused by this concept. I'm my experience with RAIDs I have only > every delt with a RAID 1 that has degraded. I simply set the drive as > offline, replaced it, set it to online and the RAID rebuilt itself all > without restart the server and operation was never interrupted. So we > can presume the server had hot swappable drives enabled yes? (It was a > hardware RAID). With a software RAID is this still achievable?Hardware RAID system almost always had hot swappable drives, esp. SCSI ones. And many of the old hardware RAID SCSI server boxes were equiped with hot swappable drive bays. For software RAID, it depends on the controller and what the driver for that controller supports. It also depends on how the drives are mounted. You *can* 'hot' [un]plug conventually mounted drives (eg remove the cover of the *running* machine and reach in and pull the data and power plugs off the disk in the *correct* order), but it is tricky (and not really recomended). It is far easier to get a hot-swap chassis. If the controller supports hot swapping AND the driver supports the controller's hot swapping, yes, you can hot swap with software RAID. With RAID 5, what you want is a 'hot' spare: an additional disk that is not part of the array, but is associated with it. What you do is 'fail' the drive you want to pull. The system will then 'rebuild' itself using the 'hot' spare. You then 'remove' the 'failed' disk from the RAID set, spin it down (requires the proper incantation, such as with sg_start from sg3_utils), pull the drive, insert a new drive, scan for the new drive (eg sg_scan), spin it up (sg_start), partition it (if necessary), and then add it as a hot spare.> > Thank you for reading. > > Regards, > James ;) > > Charles de Gaulle ?- "The better I get to know men, the more I find > myself loving dogs." > _______________________________________________ > CentOS mailing list > CentOS at centos.org > http://lists.centos.org/mailman/listinfo/centos > >-- Robert Heller -- 978-544-6933 Deepwoods Software -- Download the Model Railroad System http://www.deepsoft.com/ -- Binaries for Linux and MS-Windows heller at deepsoft.com -- http://www.deepsoft.com/ModelRailroadSystem/