[DM-MUG] Using RAID with Tiger
Matt Stanton
inflatablesoulmate at brothersofchaos.com
Mon Nov 30 02:40:39 CST 2009
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
>>
>> _______________________________________________
>> DMMUG mailing list
>> Use this Address to send mail to the list:
>> DMMUG at dmmug.org <http://mc/compose?to=DMMUG@dmmug.org>
>> Use this page to modify subscription options:
>> http://cialug.org/mailman/listinfo/dmmug
>>
>>
>>
>> _______________________________________________
>> DMMUG mailing list
>> Use this Address to send mail to the list:
>> DMMUG at dmmug.org <mailto:DMMUG at dmmug.org>
>> Use this page to modify subscription options:
>> http://cialug.org/mailman/listinfo/dmmug
>>
>>
>> ------------------------------------------------------------------------
>>
>> _______________________________________________
>> DMMUG mailing list
>> Use this Address to send mail to the list:
>> DMMUG at dmmug.org
>> Use this page to modify subscription options:
>> http://cialug.org/mailman/listinfo/dmmug
>>
>
> _______________________________________________
> DMMUG mailing list
> Use this Address to send mail to the list:
> DMMUG at dmmug.org
> Use this page to modify subscription options:
> http://cialug.org/mailman/listinfo/dmmug
>
>
More information about the DMMUG
mailing list