[DM-MUG] Using RAID with Tiger
Jon Thompson
jthompson-lists at dmevolve.com
Mon Nov 30 09:06:47 CST 2009
Considering that Software RAID is used on many a server (I have one XServe
that doesn't have a RAID card that is running Software RAID for instance) it
is fine for normal users. It does slow your computer more than a
hardware-based machine, but not enough to recommend its non-use.
--
Jon Thompson
Evolve
www.dmevolve.com
On Mon, Nov 30, 2009 at 2:40 AM, Matt Stanton <
inflatablesoulmate at brothersofchaos.com> wrote:
> Bleh, it was bound to happen, but I got the raid types confused halfway
> through the email.
>
> Where I said:
>
> RAID0 is only really useful if you need to be able to hotswap the failed
> drive. Otherwise, it makes much more sense to use two drives of the
> same size and do regular or incremental backups.
> RAID1 is dangerous, and only useful if you need to be able to read/write
> data to a hard drive much faster than a normal user.
>
>
> It should read:
>
> RAID1 is only really useful if you need to be able to hotswap the failed
> drive. Otherwise, it makes much more sense to use two drives of the
> same size and do regular or incremental backups.
> RAID0 is dangerous, and only useful if you need to be able to read/write
> data to a hard drive much faster than a normal user.
>
>
>
> Matt Stanton wrote:
> > Here are some of the pros and cons of RAID setups:
> > RAID0 (striping): 2 500GB drives == One RAID volume of 1000GB
> > pros:
> > Twice the data transfer throughput of a single HD
> > Only requires 2 HDs
> > You have the amount of space of both hard drives combined
> > cons:
> > If one drive fails, you lose all the data on both drives
> >
> > RAID1 (mirroring): 2 500GB drives == One RAID volume of 500GB
> > pros:
> > Only requires 2 HDs
> > If one drive fails, you still have all of your data
> > cons:
> > No speed boost over a single drive
> > You have only half the combined storage space of both hard drives.
> >
> > RAID5 (striping + parity): 3 500GB drives == One Raid volume of 1000GB
> > pros:
> > Twice the data transfer throughput of a single HD
> > If one drive fails, you have half your data, plus enough parity data to
> > regenerate the other half
> > cons:
> > If one drive fails, you not only lose the throughput of the second drive
> > (you drop to the throughput of a single drive), but any data that was
> > lost on the failed drive must be regenerated from the two surviving
> > drives when you access the data (this shouldn't cause an additional
> > performance drop, since the hard drives are where the speed bottlenecks,
> > but if you are using software RAID, it would mean extra processing on
> > the CPU)
> > Requires at least 3 equally-sized hard drives
> >
> > RAID10, RAID0+1, RAID1+0, RAID1&0 (striping + mirroring): 4 500GB drives
> > == One Raid volume of 1000GB
> > pros:
> > Twice the data transfer throughput of a single HD
> > Data can survive the failure of 2 HDs
> > If 1 or 2 drives fail, you retain the throughput performance of the
> > striped array
> > cons:
> > Requires 4 drives and gives you half the combined space of those 4 drives
> >
> > There are also RAID3 and RAID4, which are similar to RAID5, in that they
> > are a mirrored+parity array (they just handle the parity differently
> > than the RAID5 array does). RAID6 is a mirror+parity type of array that
> > uses 2 parity drives so that it can survive the loss of 2 drives, where
> > RAID3,4,5 would only be able to survive the loss of 1 drive.
> >
> > RAID0 is only really useful if you need to be able to hotswap the failed
> > drive. Otherwise, it makes much more sense to use two drives of the
> > same size and do regular or incremental backups.
> > RAID1 is dangerous, and only useful if you need to be able to read/write
> > data to a hard drive much faster than a normal user. It would be
> > recommended for use as the drive that the operating system and
> > applications are saved to (since you can usually reinstall the OS and
> > Applications from their original media). All data/documents should be
> > saved to a separate drive.
> > RAID5 is especially useful in a server environment, since it allows the
> > server to continue functioning even if a drive dies. Generally the
> > server supports hot-swappable drives so that when a drive fails, you
> > simply pull it out and replace it with a new drive, and the data is
> > automatically regenerated with the server still running. RAID5 is also
> > a better choice if you don't have the money for the more expensive
> > RAID10 style arrays.
> > RAID10 is the most fault-tolerant of the arrays, while also giving you
> > extra speed. This type of array is the most expensive, though, since
> > you get only half the storage space of the 4 drives.
> >
> > The only RAID type that is of much noticeable benefit on a desktop
> > computer is RAID0. Many people use a RAID0 array as a temporary storage
> > area for programs that do large amounts of data manipulation, such as
> > video editing. In the windows world, it is often used on gaming
> > computers to reduce game load time. In either case, everything on the
> > RAID0 array should be backed up to a secondary location regularly.
> >
> > All of the other raid types are used for either a mix of performance and
> > data safety or for data safety on its own. You would get the same level
> > of safety if you simply back up all of your important data regularly.
> > RAID mirroring will not protect your data against accidental deletion or
> > overwriting, which is why backups are important.
> >
> > I am a bit leery of Software RAID (which is what is present on most
> > desktop computers these days). Since the array is dependent on the OS
> > to achieve the results, you are still using processor power and sending
> > extra data over the SATA/IDE bus. Hardware RAID was/is available in
> > many Macs (at least, I have seen it available in the Mac Pro line), and
> > if you have the hardware RAID card, then you would be well-served to use
> > it (assuming you can afford the extra hard drives).
> >
> > - Matt
> >
> > Jon Thompson wrote:
> >
> >> Please note that the article Anastasia reposted to the list is
> >> available on my company website www.dmevolve.com
> >> <http://www.dmevolve.com>, as well as other articles.
> >>
> >> Anastasia has it wrong. If we're talking about support beams...
> >>
> >> - RAID1 is making sure that every beam is at 50% capacity- you can
> >> lose half and your building won't fail.
> >> - RAID0 is making sure that every beam is at 100% capacity- you can't
> >> lose a single beam without losing the building.
> >>
> >> RAID0 takes individual files, divides them in half and splits them
> >> between drives, so if you lose one drive, you've lost all of your
> >> data. This doubles the write speed of the drives, which is why it is
> >> used at all, for specific purposes, which are _always_ stored
> >> elsewhere as well.
> >>
> >> Striped is RAID 0 and like I said, increases the likelihood of data
> >> loss. Avoid it unless you know exactly why you want it. RAID 1
> >> performs the exact function of making a SuperDuper or Carbon Copy
> >> Cloner mirror of your drive. Except that it does it better, as it is
> >> in real time. It _only_ protects you from disk failure, which is
> >> actually a rarity, unless your drive is very new or very old. In the
> >> latter case, it is a better idea to get a new drive, burn it in for
> >> two weeks, then migrate your data to it using CCC.
> >>
> >> As for configuring a RAID. Disk Utility will wipe all drives when
> >> creating a RAID. However, it is possible to do it without wiping the
> >> drive using the diskutil command line application. Please do not do
> >> this unless you are comfortable with the commands and how to modify
> >> my examples to suit your needs. I am not teaching you how to get to
> >> the command line. If you don't already know, this is not the set of
> >> commands for you, as there is a very real chance to lose your data.
> >>
> >> diskutil list (pay attention to the drivenumbers, which are listed as
> >> "disk#". Most Macs have a boot drive of disk0)
> >> diskutil enableRAID mirror disk<drivenumber of main boot partition>
> >> diskutil repairMirror disk<drivenumber of main boot partition>
> >> disk<drivenumber of empty drive>
> >>
> >> The other thing is that the 1 TB drive will effectively become 500 GB,
> >> wasting the remaining space. I do not believe it is possible to
> >> partition part of a drive that is used in RAID, but could be wrong, as
> >> I _never_ use non-matching drives in RAID, like you are wanting to do
> >> here. Part of this is I/O. It is imperative that data is written to
> >> the two hard drives simultaneously, otherwise, data corrupting
> >> bottlenecks are possible. Since one hard drive is now responsible for
> >> two partitions, it no longer has 100% of it's time dedicated to
> >> maintaining the RAID.
> >>
> >> Finally, the only way that I would consider doing a software based
> >> raid is if you have two internal drives, such as in a MacPro, XServe,
> >> or the new Mac Mini Server. DO NOT DO RAID OVER FIREWIRE. (Unless you
> >> are buying a RAID-based device like a Drobo, or the better performing
> >> Promise SmartStor DS4600, which house all drives internally.) If you
> >> have one drive internal, and the other over firewire and you pull the
> >> cord out, you are going to rebuild your RAID. Same with the power
> >> cord. It's just not a good idea, as you are increasing the likelihood
> >> of compromising your redundancy.
> >> --
> >> Jon Thompson
> >> Evolve
> >> www.dmevolve.com <http://www.dmevolve.com>
> >>
> >>
> >> On Sun, Nov 29, 2009 at 7:05 PM, AB <anastasia_prittee at yahoo.com
> >> <mailto:anastasia_prittee at yahoo.com>> wrote:
> >>
> >> As I understand it...
> >> RAID Mirrored mirrors the contents and files of a single drive in
> >> the event that drive fails—see excerpts from the info Jon gave.
> >> RAID Stripped takes content and distributes it across multiple
> >> drives. Kind of like putting multiple support beams in. Each hard
> >> drive is a support beam and collectively behave as one unit. One
> >> of those drives failing won't necessarily have detrimental
> >> repercussions. I believe John Thompson gave a detailed
> >> description. Though I might be confused on this.
> >>
> >> As far as erasing contents on the existing drive, I'm not sure and
> >> was wondering the same thing myself.
> >>
> >> Here's what I saved for reference on what was mentioned before
> >> about RAID. I thought there was another detailed summary, Jon
> >> gave, but I this seems to be all I saved.
> >>
> >> Jon Thompson wrote this on May 19, 2009:
> >>
> >> *1) A good backup is a verifiable backup*
> >>
> >> If you are dumping info into Time Machine, and you are _never_
> >> pulling info back out of it and checking, how do you know that the
> >> data is good? In the instance of the SideKick data, this means
> >> that they would have known that the backup is good, and that they
> >> could rely on it. Until you know that you can recover from it, you
> >> cannot call it a good backup.
> >>
> >> *2) A good backup consists of many backups of the same data.*
> >>
> >> I store three months of backups, and I consider that too little,
> >> but don't dare increase my budget. If you are relying on all of
> >> your data being on a single hard drive, what happens if there is a
> >> fire and both your computer and backup is lost? If all of your
> >> backup is in one cloud (Mozy) and that cloud bursts like the
> >> SideKick cloud did, you are hosed. Now, if you have daily backups
> >> on a time machine volume, and a weekly backup to Mozy, you are
> >> protected from either example. Mozy is your offsite backup, the
> >> time machine
> >>
> >> *3) Synchronization is not backup.*
> >>
> >> Synchronization ensures that all of you data is the same between
> >> all instances that you create. If you are syncing your address
> >> book on your mac with an iPhone, you have two instances of the
> >> data, but they are tied together. If you delete a contact on one
> >> (or a virus deletes all of them) that deletion is transferred to
> >> your backup. I realize that there are ways that the sync service
> >> tries to reduce the chance of this, but it is still a possibility.
> >> However, an offline copy of your address book on a USB thumb drive
> >> that is not plugged into your computer cannot not be changed, at
> >> least until it is plugged in again. Dropbox is a synchronization
> >> service.
> >>
> >> *4) RAID is not backup.*
> >>
> >> RAID (except 0) is a way to protect from hard drive crashes. It
> >> does not protect from application corruption, accidental deletion,
> >> or intentional deletion. All of which are much more likely than
> >> hard drive corruption during the majority of the life of a hard
> >> drive. RAID is useful for server admins that deal with
> >> distributing data amongst many hard drives, which actually is a
> >> great way to exponentially increase the likelihood of data loss
> >> due to hard drive failure.
> >>
> >> RAID 0 offers no protection at all- in fact, it is will
> >> exponentially increase your likelihood of data loss due to hard
> >> drive failure. Be careful when purchasing large external hard
> >> drive, as they are sometimes two drives that are internally
> >> configured as RAID 0. This is useful for professional video
> primarily.
> >>
> >> *5) Backup as sparse as you are willing to recover from.*
> >>
> >> - If you couldn't stand losing more than a week's worth of data,
> >> you'd best be backing up weekly.
> >> - If you couldn't stand losing more than a day's worth of data,
> >> you'd best be backing up daily.
> >> - If you couldn't stand losing more than an hour's worth of data,
> >> you'd best be backing up hourly.
> >>
> >> You know what is important and what is not. Make sure to taylor
> >> your backup routine to your needs. Incremental backups, such as
> >> Time Machine are good about this.
> >>
> >>
> >>
> >> --- On *Sun, 11/29/09, John Robertson /<john at createmydvd.com
> >> <mailto:john at createmydvd.com>>/* wrote:
> >>
> >>
> >> From: John Robertson <john at createmydvd.com
> >> <mailto:john at createmydvd.com>>
> >> Subject: [DM-MUG] Using RAID with Tiger
> >> To: "Des Moines Mac Users Group" <dmmug at dmmug.org
> >> <mailto:dmmug at dmmug.org>>
> >> Date: Sunday, November 29, 2009, 11:52 AM
> >>
> >>
> >> Just wondering if anyone has any experience using RAID on a single
> >> user OS. I purchased a new 1TB drive and I was thinking of
> >> partitioning it to use as a mirror for my 500G drive. I have two
> >> questions before I attempt this. The 1st question is, will it erase
> >> the contents of the old drive (the 500 gig) that I am adding the 1TB
> >> to when I turn on RAID? Also, what are the differences between RAID
> >> mirrored and RAID Striped? I understand how Mirroring works and a
> >> little about RAID 5 (which is only available on servers) but I don't
> >> know which of the other option available here would be better to
> use.
> >>
> >> Thanks,
> >> John
> >>
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