topic Re: Mirror in Operating System - HP-UX

Mirror

Roberto Volsa — Tue, 25 Nov 2003 10:28:57 GMT

Simple question: for a standard mirrored LV, the writing on the two PEs is performed at the same time or there is a delay between the writing on the 2 disks?
If there is a crash; the consistency between the two copies is guaranteed?
Is it possible to change the writing politics to the 2 PEs?

Thanks a lot

Roberto

Re: Mirror

Jean-Louis Phelix — Tue, 25 Nov 2003 10:36:11 GMT

Hi,

By default, PEs are written in parallel mode. You can change it to sequential by using 'lvchange -d s'. In case of crash, lvm tries to resynchronize all PEs.

Regards.

Re: Mirror

TSaliba — Tue, 25 Nov 2003 10:36:30 GMT

hi
have a look to the following link:
http://www.docs.hp.com/cgi-bin/fsearch/framedisplay?top=/hpux/onlinedocs/B2355-90672/B2355-90672_top.html&con=/hpux/onlinedocs/B2355-90672/00/00/69-con.html&toc=/hpux/onlinedocs/B2355-90672/00/00/69-toc.html&searchterms=mirror&queryid=20031125-083348

TS

Re: Mirror

RAC_1 — Tue, 25 Nov 2003 10:37:56 GMT

I think we have some control over this. Do not have a system at hand at the moment.

If my memory serves me right, check manpage for vgcreate. It has got some option for this.

Re: Mirror

Bernhard Mueller — Tue, 25 Nov 2003 10:39:20 GMT

Roberto,

that is what the "Mirror Write Cache" is for. By default it is enabled and guarantees consistency.

It allows LVM to determine whether all the mirror copies are identical, even across system crashes.

You can switch it off with lvcreate using "-M n". Then you are save only for deactivting the vg and re-activating, not across a crash.

You can switch consistency recovery off with lvcreate using "-c n". In which case I'd have to ask you why you do mirroring at all .... ;-)

Regards,
Bernhard

Re: Mirror

John Palmer — Tue, 25 Nov 2003 10:39:29 GMT

Default is parallel writes, you can change this with the -d flag of lvcreate/lvchange.

man lvcreate for info, note the warning about performance if you set schedule to sequential!

Consistency after a crash is influenced by the -c and -M flags.

Regards,
John

Re: Mirror

RAC_1 — Tue, 25 Nov 2003 10:43:51 GMT

It is lvcreate/lvchange

man lvcreate/lvchange

Re: Mirror

A. Clay Stephenson — Tue, 25 Nov 2003 10:44:32 GMT

The default mode is parallel but you can select sequential update policy. You are trading off speed for reliability. In practice, there is very little reason to use the sequential update policy because 1) the latency introduced by the UNIX buffer cache and 2) the on-disk cache. Note that even in the case of the write being sent to disk really only means that it got to the disk's on-board buffer rather than written to the disk.

Re: Mirror

James Lynch — Wed, 26 Nov 2003 10:40:06 GMT

What Clay is referring to is "Immediate Disk Reporting". This is a feature found in most modern disk drives. For those that are really paranoid, this capability can be disabled. Disabling this feature is NOT reccomended due to the significant performance hit that results.

JL