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Mirror across options for mirrored volumes

Balthier35
Level 3

During creation of mirrored volumes, you can choose to Mirror across disks by Port, Target, Enclosure, or Channel. I am not able to find in-depth information for these options. I do understand that if you have two enclosures, and you choose to mirror across enclosure, you will create the mirror on disks located at different enclosures.

But what if you manually create a Dynamic Disk Group, which contains disks from two different enclosures. Then you create a volume on that Disk Group.
Do you still have to specify that you want to mirror across enclosures then? Because thats already what you are doing by creating a mirrored volume on a Disk Group that contains disks from two different enclosures.

What are the benefits of specifying mirror across options?

1 ACCEPTED SOLUTION

Accepted Solutions

Wally_Heim
Level 6
Employee

Hi Balthier35,

since you are only selecting 2 disks and the 2 disks are in different arrays, then each plex will be on a different array regardless of the mirror across setting.  However, SFW is not aware that this is a requirement in your environment because you did not select a mirror across option.  If you extend the volumes in the future, SFW may or may not maintain the subdisks for each plex in the correct array since you did not tell SFW that was important to you.

If you had selected the mirror across enclosures when creating the volume, SFW will maintain that when expanding the volume.  The good news is that there is an option during the volume expand screen that allows you to select the mirror across option when expanding.

If you are manually selecting the 2 disks during the initial mirror creation then the option does not need to be selected.  The option is really intended for mirrors that are made up of more than 1 disk/lun per enclosure.

thank you,

Wally

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6 REPLIES 6

Wally_Heim
Level 6
Employee

Hi Balthier35,

Yes, you will need to select the mirror across option so that SFW knows how to maintain the mirror.  If you do not select the mirror across option then SFW will assume that there is no concern with what disks are used for each plex and it will select disks from each array for a single mirror plex.  However, with the mirror across option selected, SFW will select only disks from a single array for a given plex (complete copy of the volume.)  This will ensure that the volume is available even if one of the arrays is not available.

 

Thank you,

Wally

Balthier35
Level 3

Hi Wally,

Thank you very much for your response. I have a two node failover cluster, where the nodes are placed at two different sites, lets call them SiteA and SiteB. The nodes are virtual machines running on VMware ESXi 5.1 hosts. Two data disks have been made available for the nodes. These data disks are presented from enclosures at two different sites (A and B).

In Veritas Enterprise Administrator, I can see that the two disks are from two different enclosures. So I create a VMDG, containing these two disks. Then I create a mirrored volume on the VMDG, that way I am selecting disks from two different enclosures, and even if I don't specify the option, the mirror is being made across two different enclosures.

I am not auto selecting disks, I'm manually selecting them, and specifying disks from two different enclosures. Thats why I am wondering if I still need to specify the mirror across enclosures option. Because if I click Disk View on the volume in VEA, I can see that the mirrored volume is residing evenly on two different disks (from two different enclosures).

Could you please comment on this? :)

Also, could you please elaborate on what you meant in this sentence

" If you do not select the mirror across option then SFW will assume that there is no concern with what disks are used for each plex and it will select disks from each array for a single mirror plex."

Doesn't mirrored volumes always have two (or more) plexes, regardless of how they are mirrored?

Balthier35
Level 3

This is how it looks in DiskView of the volume

clus1.png

As you can see Harddisk1  and Harddisk2 are in two different enclosures

clus2.png

I did not select the option to mirror across enclosures while creating this volume. What are the disadvantages of creating the volume in this manner? And is there any way I can rectify the disadvantages?

Wally_Heim
Level 6
Employee

Hi Balthier35,

since you are only selecting 2 disks and the 2 disks are in different arrays, then each plex will be on a different array regardless of the mirror across setting.  However, SFW is not aware that this is a requirement in your environment because you did not select a mirror across option.  If you extend the volumes in the future, SFW may or may not maintain the subdisks for each plex in the correct array since you did not tell SFW that was important to you.

If you had selected the mirror across enclosures when creating the volume, SFW will maintain that when expanding the volume.  The good news is that there is an option during the volume expand screen that allows you to select the mirror across option when expanding.

If you are manually selecting the 2 disks during the initial mirror creation then the option does not need to be selected.  The option is really intended for mirrors that are made up of more than 1 disk/lun per enclosure.

thank you,

Wally

View solution in original post

Balthier35
Level 3

Thank you very much for your thorough explanation Wally.

Btw, do you know of any source, where I can read up on this issue? I am having a difficult time finding any in-depth information related to this. I really can not find a book which deals with this issues,

Wally_Heim
Level 6
Employee

Hi Balthier35,

I'm not aware of any books other than the Guides that come with the product.  The product guides are available for download on the Sort site at: http://sort.symantec.com.

The admin guide is a good source of infromation on SFW functions.

Thank you,

Wally