[linux-lvm] What happens when a disk dies?

[linux-lvm] What happens when a disk dies?

[Kyle_S_Hoyt]

I am very interested in starting to use LVM.  We have large amounts of data
that is fairly dynamic.  Since it is dynamic, we don't need to incur the
cost expensive SCSI RAID solutions for data integrity.  We currently use
four 80 Gig Maxtor Hard Drives to store our data at test events. Of course
this means with a /drive1 /drive2 /drive3 and a /drive4 directory.  So the
user has to search the drives to find the file of interest.  We kind of
hide this by creating links but the links need to be periodically updated
as files are added or deleted.  LVM solves this problem.  But before I move
to LVM, I curious on what happens if a drive dies.

 If drive2 goes down and I replace it with another drive, do I just lose
the data on drive2 or do I lose all of the data in the Logical Volume?

If I don't lose the data on the other drives, can LVM also be reconfigured
to remove the dead drive and then continue working with just the other
three drives (only losing the data on the dead drive)?

I read the FAQs and one of them shows how to recover from a dead drive.  It
is says you lose the data in the Logical Volume but I'm hoping just the
data on the physical disk that died and not the entire volume.

Thanks for your help,

Kyle Hoyt
(727) 302-7465
Kyle_S_Hoyt@raytheon.com

Re: [linux-lvm] What happens when a disk dies?

[Adrian Phillips]

>>>>> "Kyle" == Kyle S Hoyt <Kyle_S_Hoyt@raytheon.com> writes:

    Kyle> I am very interested in starting to use LVM.  We have large
    Kyle> amounts of data that is fairly dynamic.  Since it is
    Kyle> dynamic, we don't need to incur the cost expensive SCSI RAID
    Kyle> solutions for data integrity.  We currently use four 80 Gig
    Kyle> Maxtor Hard Drives to store our data at test events. Of
    Kyle> course this means with a /drive1 /drive2 /drive3 and a
    Kyle> /drive4 directory.  So the user has to search the drives to
    Kyle> find the file of interest.  We kind of hide this by creating
    Kyle> links but the links need to be periodically updated as files
    Kyle> are added or deleted.  LVM solves this problem.  But before
    Kyle> I move to LVM, I curious on what happens if a drive dies.

    Kyle>  If drive2 goes down and I replace it with another drive, do
    Kyle> I just lose the data on drive2 or do I lose all of the data
    Kyle> in the Logical Volume?

This is a filesystem thing not LVM (well LVM is involved if you use
it). If you lose a drive the filesystem is going to be unhappy to put
it mildly.

I don't understand your problem here , you can :-

- mount each disk under /this_is_the_data as its own filesystem (not
  using LVM necessarily), say /this_is_the_data/drive<n>

- search from /this_is_the_data

- if a disk goes then umount drive<n> and searches can still continue

If this does not work then more information is needed.

Sincerely,

Adrian Phillips

-- 
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Re: [linux-lvm] What happens when a disk dies?

[Heinz J. Mauelshagen]

On Fri, May 11, 2001 at 11:16:02AM -0400, Kyle_S_Hoyt@raytheon.com wrote:
> 
> 
> I am very interested in starting to use LVM.  We have large amounts of data
> that is fairly dynamic.  Since it is dynamic, we don't need to incur the
> cost expensive SCSI RAID solutions for data integrity.  We currently use
> four 80 Gig Maxtor Hard Drives to store our data at test events. Of course
> this means with a /drive1 /drive2 /drive3 and a /drive4 directory.  So the
> user has to search the drives to find the file of interest.  We kind of
> hide this by creating links but the links need to be periodically updated
> as files are added or deleted.  LVM solves this problem.  But before I move
> to LVM, I curious on what happens if a drive dies.
> 
>  If drive2 goes down and I replace it with another drive, do I just lose
> the data on drive2 or do I lose all of the data in the Logical Volume?

You need to restore the LVM metadata onto the replacement drive in order
to activate the volume group again.
Because LVM maps the logical address space of the logical volume in units
of logical extends (LEs; typically 4M in size) to the equal sized physical
extents (PEs) on the underlying disk devices, all allocated PEs on the
replaced drive will have bad contents.
This will expose a partially messy filesystem which will give you plenty of
errors soon and won't make fsck happy at all :-(

> 
> If I don't lose the data on the other drives, can LVM also be reconfigured
> to remove the dead drive and then continue working with just the other
> three drives (only losing the data on the dead drive)?

Not today.

The feature you are requesting is known as VG quorum.

IOW: if less than a reasonable amount of physical volumes within a volume
group fail, you can activate the VG *but* you need to live with the above
mentioned fs (or database or whatever) problem. Just depends on what is stored
in the partially available LV and what complaints of LVM user software
entities (like filesystems, database systems etc.) are caused by that.

VG quorum will be supported in a later LVM version.

> 
> I read the FAQs and one of them shows how to recover from a dead drive.  It
> is says you lose the data in the Logical Volume but I'm hoping just the
> data on the physical disk that died and not the entire volume.

Yep, as explained above.

For enhanced availability you should make use of MD software RAID >= 1 or
hardware RAID subsystems.

Regards,
Heinz    -- The LVM Guy --

> 
> Thanks for your help,
> 
> Kyle Hoyt
> (727) 302-7465
> Kyle_S_Hoyt@raytheon.com
> 
> 
> _______________________________________________
> linux-lvm mailing list
> linux-lvm@sistina.com
> http://lists.sistina.com/mailman/listinfo/linux-lvm

*** Software bugs are stupid.
    Nevertheless it needs not so stupid people to solve them ***

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Heinz Mauelshagen                                 Sistina Software Inc.
Senior Consultant/Developer                       Am Sonnenhang 11
                                                  56242 Marienrachdorf
                                                  Germany
Mauelshagen@Sistina.com                           +49 2626 141200
                                                       FAX 924446
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Re: [linux-lvm] What happens when a disk dies?

[Steven Lembark]

> I am very interested in starting to use LVM.  We have large amounts of data
> that is fairly dynamic.  Since it is dynamic, we don't need to incur the
> cost expensive SCSI RAID solutions for data integrity.  We currently use
> four 80 Gig Maxtor Hard Drives to store our data at test events. Of course
> this means with a /drive1 /drive2 /drive3 and a /drive4 directory.  So the
> user has to search the drives to find the file of interest.  We kind of
> hide this by creating links but the links need to be periodically updated
> as files are added or deleted.  LVM solves this problem.  But before I move
> to LVM, I curious on what happens if a drive dies.
> 
>  If drive2 goes down and I replace it with another drive, do I just lose
> the data on drive2 or do I lose all of the data in the Logical Volume?
> 
> If I don't lose the data on the other drives, can LVM also be reconfigured
> to remove the dead drive and then continue working with just the other
> three drives (only losing the data on the dead drive)?
> 
> I read the FAQs and one of them shows how to recover from a dead drive.  It
> is says you lose the data in the Logical Volume but I'm hoping just the
> data on the physical disk that died and not the entire volume.


LVM will not replace mirroring or RAID solutions.  for lots of
dynamic data the best solution will probably be hardware RAID
for the physical layer with LVM using RAID volumes as PV's.

e.g., if you can 4 disks into a RAID5 set then take the whole
RAID volume and use the entire space as a single PV.  you can
then group multiple PV's together into a single VG.  this VG
then gets divied up for the users into various LV's which get
mounted wherever the folks are supposed to use them.  you can
expand an LV within the VG (example:  you just bought 4 more
disks and need to expand existing space into them).  

nice thing about this arrangement is that you can have huge amounts
of total space without needing to expose all of it at once or
drop and rebiuld the filesystems to grow them.  when someone
really does run out of space (or someone new needs it) just expand
or create a LV and go along with life.


-- 
 Steven Lembark                                   2930 W. Palmer St.
                                                 Chicago, IL  60647
 lembark@wrkhors.com                                   800-762-1582