Mirror Write Consistency for Logical Volumes

[olli2003]

Dear all,

I've some trouble with Logical Volumes for data files!
For this, can someone tell me, if it's a problem to
switch this point (Mirror Write Consistency) from "YES"
to "NO"? I've read that after switching it will
comes to problems f. ex. by restoring a database after
a system crash, and that there're special things
you've to managed before this!

Thanks in advance!

Regards
Oliver

 

[Morsing]

MWC works like a log file for the mirror. After a crash the LVM will pick i PV as a source PV to update the other mirrors from by writing the last transactions from the MWC to th PVs.

Don't know if you have to do anything before??

If you want high reliability use sequential write (parallel write set to no). That way you won't need MWC.

What kind of problems do you have??

Cheers Henrik Morsing
Certified AIX 4.3 Systems Administration
& p690 Technical Support

 

[norafh]

Hello Henrik,

thanks for your reply! Yes, sure, but if I'll do so, my performance will shrink in any case! So that's what I don't want. We've problems in performance, so we'll
choose parallel reading/writing for the LV, but we
also want to set the MWC to "NO". I don't know the note wich tells me what to do in such a case.

Best Regards
Oliver

 

[ElgisRamon]

What version are you on? AIX 4 has MWC yes/no and AIX 5 has active/passive/off...
I hope it works...
Unix was made by and for smart people.

 

[Morsing]

Technically speaking, parallel sched policy without MWC only works if a disk dies during normal operation. That way the other disk will just be marked as stale.
If there's a power cut or the adapter dies you wont know for sure what is missing.

Cheers Henrik Morsing
Certified AIX 4.3 Systems Administration
& p690 Technical Support

 

[Chapter11]

another trick to improve performance when using mirrors and mwc = on is to use location policy to place mirrored LVs at the outer edges of disks. the mwclog is located at the outer edge and is not relocatable, so by putting mirrored lvs at the outer edge, you improve performance by minimizing head travel.

 

[olli2003]

Thanks for all replies! I'll think about all positive or negative characters before changing the status of our AIX 4.3.3 system.

Best Regards
Oliver