Looking For RAID Hardware/Software Advice v1

[Lisa Kachold]

Thanks!

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> Date: Mon, 19 Jan 2009 22:54:18 -0700
> From: cryptworks at gmail.com
> To: plug-discuss at lists.plug.phoenix.az.us
> Subject: Re: Looking For RAID Hardware/Software Advice v1
> 
> That was very detailed
> And quite good
> 
> 
> On 1/19/09, Lisa Kachold <lisakachold at obnosis.com> wrote:
> >
> >
> >
> >
> >
> >
> >
> > 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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> >
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> >
> >
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> -- 
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> 
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> rolling over and going back to sleep after you hit the snooze button.
> 
> Stephen
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