What is the purpose of synchronization in RAID 1 mirroring?

[chrisk]

I've searched for an answer to this question but couldn't come up with
anything definitive. Obviously it has to do with making sure that the
two disks in a RAID 1 configuration are identical, and synchronization
corrects errors and inconsistencies, but why would the two drives not
be identical in the first place?

In my opinion, if the two drives are not absolutely identical at all
times, that makes the RAID adapter pretty much worthless and the
mirror a non-mirror. . If I have to manually synchronize once a month,
week, or day to make sure that the drives remain identical, I might as
well be running a backup program to another hard drive. Hell, it might
even be faster because synchronization sure takes a while.

I have a Promise FastTrak100 TX2, which I believe is considered a
software raid card. Does anyone know if you also have to synchronize
RAID 1 mirrors on true hardware RAID adapters?

chris

 

[Ronald]

I've searched for an answer to this question but couldn't come up with
anything definitive. Obviously it has to do with making sure that the
two disks in a RAID 1 configuration are identical, and synchronization
corrects errors and inconsistencies, but why would the two drives not
be identical in the first place?
Agreed
In my opinion, if the two drives are not absolutely identical at all
times, that makes the RAID adapter pretty much worthless and the
mirror a non-mirror. . If I have to manually synchronize once a month,
week, or day to make sure that the drives remain identical, I might as
well be running a backup program to another hard drive. Hell, it might
even be faster because synchronization sure takes a while.

I can see no need for a periodical re-sync either, the re-synch option can
be useful though.
Say you want to upgrade to a new version of your OS. What you could do is
break the mirror manually, install the new OS on one disc and test for a
week orso. If you want to keep your new OS, you can re-sync normally, if you
dont like it you can re-sync the 'wrong' way and get your old OS back.

I have a Promise FastTrak100 TX2, which I believe is considered a
software raid card. Does anyone know if you also have to synchronize
RAID 1 mirrors on true hardware RAID adapters?

chris

Shouldn't be any different. I checked the Promise website, couldnt find
anything on periodical re-synching.

Ronald

 

[Marc Reinig]

The reason you are using RAID is to protect against bad data being stored on
or retrieved from your hard drive or having a hard drive fail. If you
weren't worried you wouldn't use it. If you find an error on your drive,
the other drive is used.

But your other drive could have bad data. You protect against that by
periodically resync'ing them so your back up is also guaranteed good, at
least as of the last back up instead of continuing to lose data undetected
until it is called upon to retrieve it because the first drive failed.

It is safety upon safety. If you are already paranoid, then this is a small
price to pay for the really paranoid.

Marc Reinig
System Solutions

 

[chrisk]

The reason you are using RAID is to protect against bad data being stored on
or retrieved from your hard drive or having a hard drive fail. If you
weren't worried you wouldn't use it. If you find an error on your drive,
the other drive is used.

The reason why I use raid 1 is to protect myself against hard drive
failure. The protection against bad data is news to me. If I have a
corrupted file for instance, then that file should be perfectly
corrupted the same way on both drives, should it not?

I have a feeling I'm going to need to read a white paper or book about
this topic.

But your other drive could have bad data. You protect against that by
periodically resync'ing them so your back up is also guaranteed good, at
least as of the last back up instead of continuing to lose data undetected
until it is called upon to retrieve it because the first drive failed.

That's the thing. The data can be different on the two drives but the
status of the mirror still remains "functional" or "OK" when in
reality it's not. Isn't it the job of the RAID controller to
automatically fix inconsistencies (or at least automatically inform me
about them) when the data on the two drives is not the same? Or is
this only true with the more expensive "hardware raid" devices that
have onboard processors?

chris

 

[Marc Reinig]

You need to read more on RAID. There are at least 5 types. Mirroring trys
to duplicate the writes to one on another. When you read, you read from
your primary. If there is an error, hopefully it is available undamaged on
the other.

When you write, there is no guarantee you wrote correctly, that the place
where your data was correctly written did not get damaged later, or that
when you read the data it is correct. This is true on both the primary and
the mirror.

There is another type of RAID where data is distributed over several drives,
including error correcting information. In this situation, you could pull
an entire drive or volume and the data would be corrected on the fly.

Mirroring is like making a constant back up of your system. However, as
with a backup, there is no guarantee will not be damaged between the time
you made it and the time you went to use it.

For backups, normally you make incremental backups after a complete backup.
However, even there, you are advised to occasionally make a complete back up
again for the same reason you synchronize Mirror volumes occasionally.

Marc Reinig
System Solutions

 

[abhay7]

The significance of sync in raid 1 is, when we have data on the disk1 and its now been mirrored to disk2 and at the same time the disk2 is plugged out for a while and plugged in again. After its plugged in and raid1 is reassembled.
Now disk1 is ahead with production data and disk 2 has stale data, here rebuild is not required as disk 2 only has missed on few updates which it can recover from disk1.
In this scenario sync/resync comes in picture which uses write intent bitmap to know which are the blocks those need to be updated from other disk and be in mirroring state again.

Hope, I have answered your question