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Re: Duplicate lvol

 

Duplicate lvol

Hi,

We are planning to move 1TB of data in lvols of 2GB and we would like to be sure that we do it as fast as possible (we cannot use mirrors).

We are plannning to use dd with a block size of 4MB (one PE and multiple of 512b) and the raw lvol.

Is the raw device the best idea?

Thanks in advance.

Best regards
jose
7 REPLIES 7
A. Clay Stephenson
Acclaimed Contributor

Re: Duplicate lvol

There is probably no advantage to a blocksize greater than 1MB because the OS will silently break the reads and writes into smaller physical i/o's (1MB is the maximum physical io size of most versions of HP-UX). Make sure that you specify ONLY a bs= rather than an ibs= and obs= because the latter seemingly equivalent version does a buffer copy. Dd is your only choice if these are raw volumes.
If it ain't broke, I can fix that.
Bill Hassell
Honored Contributor

Re: Duplicate lvol

Yes, dd will be the fastest. You probably won't see much difference between 512k and 4Mb for the blocksize since HP-UX will breakup the I/O into smaller pieces. The break point depedns on the version of HP-UX and in some cases, the patches installed. Don't worry about 4Mb being too large--the driver will handle the large records transparently.

Yes, always use the raw device. The 'cooked' (aka, blocked) device file will go through the buffer cache, something you do not want at all. Note that mirroring would very, very slow. When you mirror an lvol, the task is done one extent at a time and every possible step is taken to ensure that the data is read correctly and then written to the mirror correctly. And during each extent mirror, all attempts to change the extent are put in a queue and will not be honored until the extent has been verified correct.


Bill Hassell, sysadmin

Re: Duplicate lvol


Thanks for the information.

We are using HP-UX 11i (11.11), do you know for that version which is the maximum physical io size?
Yogeeraj_1
Honored Contributor

Re: Duplicate lvol

hi,

1MB is the Maximum Physical I/O size of most versions of HP-UX

regards
yogeeraj
No person was ever honoured for what he received. Honour has been the reward for what he gave (clavin coolidge)
Yogeeraj_1
Honored Contributor

Re: Duplicate lvol

hi again

see also: http://docs.hp.com/en/B3921-90010/physical_io_buffers.5.html

regards
yogeeraj
No person was ever honoured for what he received. Honour has been the reward for what he gave (clavin coolidge)

Re: Duplicate lvol


Thanks for the messages.

We called HP and we got the following:
"...we have a limitation of 256k in all cases except the case of accessing disk block device bypassing LVM layer where the limitation is 1Mb.
Note that if customer uses striping in LVM - then the limit is more severe - stripe size might be less than 256k..."

Best regards,
jose

Re: Duplicate lvol


Thanks