topic Re: it is supported to mirror raw device ? in Operating System - HP-UX

it is supported to mirror raw device ?

Joël Techer — Tue, 02 Oct 2007 01:02:12 GMT

hi,

i wonder what is the result of mirroring raw device,
I've already searched any doc but i 've haven't found a lot

i did a simple test
*i had created one VG with two pv (PVG)
lvcreate + lvextend :

the following syntax is not allowed for raw device:
lvextend -m 1 /dev/vg01/lvtest /dev/rdsk/cxtxtz

the command works fine with device in block mode:
lvextend -m 1 /dev/vg01/lvtest /dev/dsk/cxtxtz

1) i don't know if my data are really the same
because mirroring raw device with disk in block mode ... ???? I've found anything about it

2) it is supported ?

the only thing i know is that raw device allow to not use buffer cache to access data faster

thank you in advance.

Jean-Louis

Re: it is supported to mirror raw device ?

Torsten. — Tue, 02 Oct 2007 01:10:05 GMT

/usr/sbin/lvextend -m mirror_copies} lv_path [pv_path ... ]

See the man page:

pv_path The block device path name of a physical volume.

This is the rule.

Re: it is supported to mirror raw device ?

Aneesh Mohan — Tue, 02 Oct 2007 01:54:09 GMT

Hi ,

DO lvsplit and confirm the data ...

Aneesh

Re: it is supported to mirror raw device ?

Matti_Kurkela — Tue, 02 Oct 2007 02:21:29 GMT

Note that each LV always has its own raw device.
For example, your /dev/vg01/lvtest has a matching /dev/vg01/rlvtest for raw access.

The "block" and "raw" device entries are just different access methods for the same areas of disk. The first uses the buffer cache and makes some assumptions to try to optimize the data access for typical filesystem use cases. The second is for efficient implementation of applications that have non-filesystem-like data access patterns. The assumption here is that the application is smart enough to do its own caching, if needed.

The lvextend command always requires you to specify the block device(s), because of the way the command is designed. However, once you've extended the lvol, the new capacity is available through the raw lvol device too.

If your application stores its data using the /dev/vg01/rlvtest device, mirroring should work just fine. This is supported.

BUT if you're configured your application to use /dev/rdsk/cxtxdz style devices, you've already bypassed the LVM layer that does the mirroring. If you establish the mirror and keep using /dev/rdsk/cxtxdz, you're effectively corrupting one half of the mirror.

MK

Re: it is supported to mirror raw device ?

Joël Techer — Tue, 02 Oct 2007 03:34:15 GMT

thank you all for your response.

Best regards

Re: it is supported to mirror raw device ?

Joël Techer — Tue, 02 Oct 2007 04:33:38 GMT

thank you