Looking For RAID Hardware/Software Advice v1

[Stephen]

That was very detailed
And quite good


On 1/19/09, Lisa Kachold <lisakachold at obnosis.com> wrote:
>
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>
>
> I have built and maintained linux production servers under both hardware
> RAID 5 on HP MSA's,
> and hardware RAID 1+0 on both HP Proliant's and Dell 2950's/1950's (with a
> variety of software disk management [depending on the server farm
> standards]).
>
> I have also configured LVM and md under SATA or iSCSI systems in both RAID 5
> and RAID 10.
>
> [I have also built Solaris servers under RAID 5/RAID 10 using SVM, and later
> replaced with N1 over multi-path I/O on Sun 2450's (zfs); and worked with
> NetApps/Redhat (xfs).]
>
> I am a proponent of md/LVM over hardware RAID because Linux md does not
> handle bad block relocation; I love simply rebuilding the array, especially
> since drives heat up, get torged by power, and are simply not build with QA
> these days.
>
> Since disk is so incredibly cheap these days (by comparison in your lower
> level [non enterprise] solution), the popularity of RAID 5 is offset by the
> full mirror and rebuild protection of RAID 1+0.
>
> Here's the technical descriptions of each level:
>
> RAID 0 is not redundant at all but offers the best
> throughput of all levels here. Data is striped across a number of
> drives so read and write operations take place in parallel across
> all drives. On the other hand if a single drive fail then
> everything is lost. Did I mention backups?
> RAID 1 is the most primitive method of obtaining redundancy
> by duplicating data across all drives. Naturally this is
> massively wasteful but you get one substantial advantage which is
> fast access.
> The drive that access the data first wins. Transfers
> are not any faster than for a single drive, even though you might
> get some faster read transfers by using one track reading per
> drive.
>
> Also if you have only 2 drives this is the only method of achieving
> redundancy.
> RAID 2 and 4 are not so common and are not covered
> here.
> RAID 3 uses a number of disks (at least 2) to store data
> in a striped RAID 0 fashion. It also uses an additional redundancy
> disk to store the XOR sum of the data from the data disks. Should
> the redundancy disk fail, the system can continue to operate as if
> nothing happened. Should any single data disk fail the system can
> compute the data on this disk from the information on the redundancy
> disk and all remaining disks. Any double fault will bring the whole
> RAID set off-line.
>
> RAID 3 makes sense only with at least 2 data disks (3 disks
> including the redundancy disk). Theoretically there is no limit for
> the number of disks in the set, but the probability of a fault
> increases with the number of disks in the RAID set. Usually the
> upper limit is 5 to 7 disks in a single RAID set.
>
> Since RAID 3 stores all redundancy information on a dedicated disk
> and since this information has to be updated whenever a write to any
> data disk occurs, the overall write speed of a RAID 3 set is limited
> by the write speed of the redundancy disk. This, too, is a limit for
> the number of disks in a RAID set. The overall read speed of a RAID
> 3 set with all data disks up and running is that of a RAID 0 set
> with that number of data disks. If the set has to reconstruct data
> stored on a failed disk from redundant information, the performance
> will be severely limited: All disks in the set have to be read and
> XOR-ed to compute the missing information.
> RAID 5 is just like RAID 3, but the redundancy
> information is spread on all disks of the RAID set. This improves
> write performance, because load is distributed more evenly between
> all available disks. Parity data is rotated across all disks so
> total net storage equals all disks minus 1.
> RAID 6 is similar to RAID 5 except that there is twice the
> redundancy and the array can survive 2 failed drives.
> Parity data is also rotated across all disks so
> total net storage equals all disks minus 2.
>
>
> There are also hybrids available based on RAID 0 or 1 and one other
> level. Many combinations are possible but I have only seen a few
> referred to. These are more complex than the above mentioned
> RAID levels.
> RAID 01 combines striping with duplication
> as mirrored arrays of striped arrays
> which gives very high transfers combined with fast seeks as well as
> redundancy. The disadvantage is high disk consumption as well as
> the above mentioned complexity.
> Also a single disk failure turns the array into RAID 0.
> RAID 1+0 combines striping with duplication
> as striped arrays of mirrored arrays
> which gives very high transfers combined with fast seeks as well as
> redundancy. The disadvantage is high disk consumption as well as
> the above mentioned complexity.
>
> When you are scrounging disk against money you are sure that two of your
> disks are not going down, so choose not to use 4 disk 1+0.  But believe me,
> THEY DO, and that's it!   Also RAID 1+0 out performs in a fast good way.
> What use is a huge cheap slow system?
>
> Full discussion available here that includes a complete analysis of md
> systems, and all the types of data loss that generally occur.
>
> http://linas.org/linux/raid.html
>
> www.Obnosis.com |  http://wiki.obnosis.com | http://hackfest.obnosis.com
> (503)754-4452
> PLUG HACKFESTS - http://uat.edu Second Saturday of Each Month Noon - 3PM
>
> Date: Mon, 19 Jan 2009 07:30:08 -0700
> From: mark at phillipsmarketing.biz
> To: plug-discuss at lists.plug.phoenix.az.us
> Subject: Re: Looking For RAID Hardware/Software Advice
>
> Eric
> Thanks for the summary, and thank-you to everyone for their ideas.
> Based on NewEgg prices, here is some more information:
>
> Option A
> Single Disk IDE Drive - 500 GB and backups, keep OS on existing drive =
> $69.99
>
> Use existing controller and just add another drive. No redundancy
>
> Option B
> RAID10 with 500 GB backup capacity and redundancy, keep OS on existing drive
> = $179.97
> 2 500 GB SATA2 Drives, new SATA2 controller
>
>
> Option C
> RAID10 with 750 GB backup capacity and redundancy, keep OS on existing drive
> = $239.97
> Two 750 GB SATA2 Drives, new SATA2 controller
>
> Option D
> RAID5 with 1,000 GB backup capacity and redundancy, keep OS on existing
> drive = $239.97
>
> Three 500 GB SATA2 Drives, new SATA2 controller
>
> I am leaning towards Option C based on less power consumption with fewer
> drives. However, I have to rethink my budget...
> After some more reading, I am a little confused about the debate between
> RAID5 and RIAD10. I am interested in the group's opinions on which is better
> - RAID 5 or RAID 10 and why? What experiences have you had regarding
> installation, maintenance, and fixing problems? I am running Debian testing.
>
> Thanks!
> Mark
> On Sat, Jan 17, 2009 at 6:35 AM, Eric Shubert <ejs at shubes.net> wrote:
>
> Mark Phillips wrote:
>
>> I am running out of room for my backups. I use backuppc and I have
>
>> almost filled a 150GB drive with backups from 7 computers, and I need to
>
>> add another 2 computers to the set. I have an old Dell Poweredge 1300
>
>> server (Pentium III 550 Mhz, 500 MB RAM, PCI 33.3Mhz) that I could turn
>
>> into a backup server. I am looking for suggestions/thoughts on how to
>
>> set this up. I need to keep the cost down as much as possible; under $150.
>
>>
>
>> My initial thoughts:
>
>>
>
>> * Keep current 72 GB drive for OS (debian testing, about 68% full)
>
>> * Add two 500 GB SATA drives and a PCI SATA controller ~$130
>
>> * Software RAID and LVM for the two drives
>
>> * Move current 150 GB of backups to the RAID
>
>> * Backuppc now runs on this machine and slowly fills up the RAID
>
>>
>
>> My questions:
>
>>
>
>> 1. Should I keep the 72 GB drive for OS, or put it on the RAID?
>
>>
>
>> 2. I can add another CPU (P III 550 MHz) processor to the box - is it
>
>> worth the effort to find one? I found one source for $5/CPU, I just need
>
>> to find the heat sink and mounting hardware. Will this improve
>> performance?
>
>>
>
>> 3. The box has a built-in SCSI 68-pin Ultra2/wide bus/controller, but
>
>> SCSI drives are more expensive, at least from a cursory google search.
>
>> Is this correct? I don't think I can use SCSI drives within my budget
>
>> constraint.
>
>>
>
>> 4. Would upgrading the memory to 1GB improve performance - top shows:
>
>> Mem: 646676k total,      639300k used     7376k free,      64548k buffers
>
>> This would add another ~$60 to my cost.
>
>>
>
>> 5. Should I look at hardware RAID cards - they seem very cheap, so
>
>> perhaps software is better?
>
>>
>
>> 4. Does this plan make sense, or is there a better way to proceed for
>
>> about the same cost?
>
>>
>
>> Thanks!
>
>>
>
>> Mark
>
>>
>
>
>
> Good replies, all. To sum things up, I think a SATAII PCI card (2 or 4
>
> port) and 2 drives is all the HW you need to add to the backup box you
>
> currently have. Set up the drives with SW RAID-1 (mirrored) and you're
>
> good to go. Migrate the data to the raid device, and keep the OS on the
>
> existing drive.
>
>
>
> With KeepItSimpleStupid in mind, I recommend using RAID-1 as opposed to
>
> RAID-5. With the price of drives these days, the additional space you
>
> get with RAID-5 isn't worth the headache you'll get when there's a
>
> problem. With RAID-1, each drive can be mounted (and used) individually
>
> if necessary. Not so with RAID-5.
>
>
>
> --
>
> -Eric 'shubes'
>
>
>
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