[linux-lvm] Software raid on top of lvm logical volume

[linux-lvm] Software raid on top of lvm logical volume

[Eric Monjoin]

Hi,

I would like to know if it's possible (works perfectly) to create a 
software mirror (md0) on top of  2 LVM logical volumes :

Physical Devices : /dev/sdb1, /dev/sdc1, /dev/sdd1, /dev/sde1, 
/dev/sdf1, /dev/sdg1
pvcreate /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1 /dev/sdf1 /dev/sdg1
vgcreate -s 32 vgdata1 /dev/sdb1 /dev/sdc1 /dev/sdd1
lvcreate -l xxx vgdata1 lvdata1
vgcreate -s 32 vgdata2 /dev/sde1 /dev/sdf1 /dev/sdg1
lvcreate -l xxx vgdata2 lvdata2

/etc/raidtab :
raiddev /dev/md0
    raid-level                     1
    nr-raid-disks                2
    nr-spare-disks              0
    chunk-size                    32
    persistant-superblock   1
    device                          /dev/vgdata1
    raid-disk                       0
    device                          /dev/vgdata2
    raid-disk                       1

Tks


Eric Monjoin
IT Architect
IBM france

Re: [linux-lvm] Software raid on top of lvm logical volume

[Theo Van Dinter]

[-- Attachment #1: Type: text/plain, Size: 512 bytes --]

On Thu, Oct 28, 2004 at 12:02:06AM +0200, Eric Monjoin wrote:
> I would like to know if it's possible (works perfectly) to create a 
> software mirror (md0) on top of  2 LVM logical volumes :

You'd usually want to make your raid devices first, then put LVM on
top of it.  I can't really think of any benefits of doing it the other
way around.

-- 
Randomly Generated Tagline:
Sarchasm: The gulf between the author of sarcastic wit, and the recipient
 who doesn't get it.
         - Washington Post

[-- Attachment #2: Type: application/pgp-signature, Size: 189 bytes --]

Re: [linux-lvm] Software raid on top of lvm logical volume

[Eric Monjoin]

Theo Van Dinter a �crit :

Well it's because we have problems in this way. We have a server 
connected to 2 EMC Symmetrix where we assign some 70Gb and 40Gb Luns.  
We used Powerpath to manage the dual path to the Luns and so I first 
created mirror as this :

raiddev /dev/md0
        raid-level              1
        nr-raid-disks           2
        nr-spare-disks          0
        chunk-size              32
        persistent-superblock   1
        device                  /dev/emcpowera1
        raid-disk               0
        device                  /dev/emcpowerf1
        raid-disk                1
#        failed-disk        1


raiddev /dev/md1
        raid-level              1
        nr-raid-disks           2
        nr-spare-disks          0
        chunk-size              32
        persistent-superblock   1
        device                  /dev/emcpowerb1
        raid-disk               0
        device                  /dev/emcpowerg1
        raid-disk                1
#    failed-disk        1

raiddev /dev/md2
        raid-level              1
        nr-raid-disks           2
        nr-spare-disks          0
        chunk-size              32
        persistent-superblock   1
        device                  /dev/emcpowerc1
        raid-disk               0
        device                  /dev/emcpowerh1
        raid-disk                1
#    failed-disk        1


raiddev /dev/md3
        raid-level              1
        nr-raid-disks           2
        nr-spare-disks          0
        chunk-size              32
        persistent-superblock   1
        device                  /dev/emcpowerd1
        raid-disk               0
        device                  /dev/emcpoweri1
    raid-disk                1
#        failed-disk        1
......
up to raiddev /dev/md9

So the /proc/mdstat  give :
Personalities : [raid1]
read_ahead 1024 sectors
Event: 15                 
md9 : active raid1 emcpowerd1[1] emcpowero1[0]
      42829184 blocks [2/2] [UU]
     
md8 : active raid1 emcpowerc1[1] emcpowern1[0]
      42829184 blocks [2/2] [UU]
     
md7 : active raid1 emcpowerb1[1] emcpowerm1[0]
      42829184 blocks [2/2] [UU]
     
md6 : active raid1 emcpowera1[1] emcpowerl1[0]
      42829184 blocks [2/2] [UU]
     
md5 : active raid1 emcpowerp1[1] emcpowerk1[0]
      42829184 blocks [2/2] [UU]
     
md4 : active raid1 emcpowerj1[1] emcpowere1[0]
      71384704 blocks [2/2] [UU]
     
md3 : active raid1 emcpoweri1[1] emcpowerd1[0]
      71384704 blocks [2/2] [UU]
     
md2 : active raid1 emcpowerc1[0] emcpowerh1[1]
      71384704 blocks [2/2] [UU]
     
md1 : active raid1 emcpowerg1[1] emcpowerb1[0]
      71384704 blocks [2/2] [UU]
     
md0 : active raid1 emcpowerf1[1] emcpowera1[0]
      71384704 blocks [2/2] [UU]
     
unused devices: <none>

But after a while I obtain that :
Personalities : [raid1]
read_ahead 1024 sectors
Event: 10                 
md9 : active raid1 [dev e9:31][1] [dev e8:e1][0]
      42829184 blocks [2/2] [UU]
     
md8 : active raid1 [dev e9:21][1] [dev e8:d1][0]
      42829184 blocks [2/2] [UU]
     
md7 : active raid1 [dev e9:11][1] [dev e8:c1][0]
      42829184 blocks [2/2] [UU]
     
md6 : active raid1 [dev e9:01][1] [dev e8:b1][0]
      42829184 blocks [2/2] [UU]
     
md5 : active raid1 [dev e8:f1][1] [dev e8:a1][0]
      42829184 blocks [2/2] [UU]
     
md4 : active raid1 [dev e8:91][1] [dev e8:41][0]
      71384704 blocks [2/2] [UU]
     
md3 : active raid1 [dev e8:81][1] [dev e8:31][0]
      71384704 blocks [2/2] [UU]
     
md2 : active raid1 [dev e8:71][1] [dev e8:21][0]
      71384704 blocks [2/2] [UU]
     
md1 : active raid1 [dev e8:61][1] [dev e8:11][0]
      71384704 blocks [2/2] [UU]
     
md0 : active raid1 [dev e8:51][1] [dev e8:01][0]
      71384704 blocks [2/2] [UU]
     
unused devices: <none>

and if we try to rebuild the mirror after after loosing access to one of 
the EMC, we have really bad result :
Personalities : [raid1]
read_ahead 1024 sectors
Event: 26                 
md9 : active raid1 emcpowerd1[2] [dev e8:e1][0]
      42829184 blocks [2/1] [U_]
      [>....................]  recovery =  1.4% (630168/42829184) 
finish=68.1min speed=10315K/sec
md8 : active raid1 emcpowerc1[2] [dev e8:d1][0]
      42829184 blocks [2/1] [U_]
     
md7 : active raid1 emcpowerb1[2] [dev e8:c1][0]
      42829184 blocks [2/1] [U_]
     
md6 : active raid1 emcpowera1[2] [dev e8:b1][0]
      42829184 blocks [2/1] [U_]
     
md5 : active raid1 emcpowerp1[2] [dev e8:a1][0]
      42829184 blocks [2/1] [U_]
     
md4 : active raid1 emcpowerj1[2] [dev e8:41][0]
      71384704 blocks [2/1] [U_]
     
md3 : active raid1 emcpoweri1[2] [dev e8:31][0]
      71384704 blocks [2/1] [U_]
     
md2 : active raid1 emcpowerh1[2] [dev e8:21][0]
      71384704 blocks [2/1] [U_]
     
md1 : active raid1 emcpowerg1[2] [dev e8:11][0]
      71384704 blocks [2/1] [U_]
     
md0 : active raid1 emcpowerf1[2] [dev e8:01][0]
      71384704 blocks [2/1] [U_]
     

So may be it will be better to create a raid device on top of the lvm 
volume.


>On Thu, Oct 28, 2004 at 12:02:06AM +0200, Eric Monjoin wrote:
>  
>
>>I would like to know if it's possible (works perfectly) to create a 
>>software mirror (md0) on top of  2 LVM logical volumes :
>>    
>>
>
>You'd usually want to make your raid devices first, then put LVM on
>top of it.  I can't really think of any benefits of doing it the other
>way around.
>
>  
>
>------------------------------------------------------------------------
>
>_______________________________________________
>linux-lvm mailing list
>linux-lvm@redhat.com
>https://www.redhat.com/mailman/listinfo/linux-lvm
>read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>

Re: [linux-lvm] Software raid on top of lvm logical volume

[Luca Berra]

On Thu, Oct 28, 2004 at 08:01:27AM +0200, Eric Monjoin wrote:
>Theo Van Dinter a �crit :
>
>Well it's because we have problems in this way. We have a server 
>connected to 2 EMC Symmetrix where we assign some 70Gb and 40Gb Luns.  
>We used Powerpath to manage the dual path to the Luns and so I first 
>created mirror as this :


>But after a while I obtain that :
>Personalities : [raid1]
>read_ahead 1024 sectors
>Event: 10                 
>md9 : active raid1 [dev e9:31][1] [dev e8:e1][0]
>      42829184 blocks [2/2] [UU]
>and if we try to rebuild the mirror after after loosing access to one of 
this might be a problem with powerpath, it seems harmless tough

>the EMC, we have really bad result :
>Personalities : [raid1]
>read_ahead 1024 sectors
>Event: 26                 
>md9 : active raid1 emcpowerd1[2] [dev e8:e1][0]
>      42829184 blocks [2/1] [U_]
>      [>....................]  recovery =  1.4% (630168/42829184) 
>finish=68.1min speed=10315K/sec
this is a problem with linux md raid. it does not support fast resync
with a bitmap of changed sectors, there is a project to implement it,
but it is not yet in the standard kernel.
look on the linux-raid mailing list archives for something called 'fast
raid 1'.
i do not believe you would have any advantage in stacking md above lvm
anyway.
Regards,
Luca

-- 
Luca Berra -- bluca@comedia.it
        Communication Media & Services S.r.l.
 /"\
 \ /     ASCII RIBBON CAMPAIGN
  X        AGAINST HTML MAIL
 / \

Re: [linux-lvm] Software raid on top of lvm logical volume

[Michael T. Babcock]

Theo Van Dinter wrote:

>On Thu, Oct 28, 2004 at 12:02:06AM +0200, Eric Monjoin wrote:
>  
>
>>I would like to know if it's possible (works perfectly) to create a 
>>software mirror (md0) on top of  2 LVM logical volumes :
>>    
>>
>
>You'd usually want to make your raid devices first, then put LVM on
>top of it.  I can't really think of any benefits of doing it the other
>way around.
>  
>

I've thought about this numerous times -- there is the distinct resizing 
advantage.  Namely, if I create a software RAID partition, I can't 
resize it afterward without destroying it.  I have for example, on 
occasion, had three disks set up where 1/3 of each was devoted to a 
RAID-0 very fast striping set for data transfers that had to be fast but 
if they were lost it wasn't critical, and 2/3 was set up as RAID-5 for 
reliability of another set of data.

I understand this may not be optimal in some situations, but some of us 
don't have thousands of dollars for SCSI HW RAID controllers.
-- 
Michael T. Babcock

[linux-lvm] Re: Software raid on top of lvm logical volume

[Peter T. Breuer]

Luca Berra <bluca@comedia.it> wrote:
> On Thu, Oct 28, 2004 at 08:01:27AM +0200, Eric Monjoin wrote:
> >md9 : active raid1 emcpowerd1[2] [dev e8:e1][0]
> >      42829184 blocks [2/1] [U_]
> >      [>....................]  recovery =  1.4% (630168/42829184) 
> >finish=68.1min speed=10315K/sec
> this is a problem with linux md raid. it does not support fast resync
> with a bitmap of changed sectors, there is a project to implement it,
> but it is not yet in the standard kernel.
> look on the linux-raid mailing list archives for something called 'fast
> raid 1'.

   fr1.sf.net (also fr5.sf.net may be of interest).

   and http://www.it.uc3m.es/ptb/fr1

(yes, I am the author). Those are for 2.4 kernels. A version fo fr1
for 2.6 kernels with additional ondisk bitmaps has been done by Paul
Clements, the linux nbd maintainer, and patches by him are around.

I should look them up and merge the two .. bids accepted!

> i do not believe you would have any advantage in stacking md above lvm
> anyway.

That is right. In fact it sounds downright risky to me.

Peter

[linux-lvm] Re: Software raid on top of lvm logical volume

[Peter T. Breuer]

Michael T. Babcock <mike@mikebabcock.ca> wrote:
> Theo Van Dinter wrote:
> 
> >On Thu, Oct 28, 2004 at 12:02:06AM +0200, Eric Monjoin wrote:
> >  
> >
> >>I would like to know if it's possible (works perfectly) to create a 
> >>software mirror (md0) on top of  2 LVM logical volumes :
> >>    
> >>
> >
> >You'd usually want to make your raid devices first, then put LVM on
> >top of it.  I can't really think of any benefits of doing it the other
> >way around.
> >  
> >
> 
> I've thought about this numerous times -- there is the distinct resizing 
> advantage.  Namely, if I create a software RAID partition, I can't 
> resize it afterward without destroying it.  I have for example, on 
> occasion, had three disks set up where 1/3 of each was devoted to a 
> RAID-0 very fast striping set for data transfers that had to be fast but 
> if they were lost it wasn't critical, and 2/3 was set up as RAID-5 for 
> reliability of another set of data.

That would be LVM over raid, not the other way round. You make a raid0
out of 1/3 of each disk (set up a partition on each) nd a raid5 out of
2/3 of each disk. Then you combine and/or divvy up the two raid areas
using lvm.

If you have lvm under raid, you STILL can't change the size of the
raided device without remaking it completely, so what's the point?  (or
does lvm somehow magically permit raid0 resizing on the fly above it -
that would be just about possible, but would require cooperation).

Peter

[linux-lvm] What is the best way to configure LVM + RAID?

[Erik Ohrnberger]

OK, so like what is the best way to configure multiple inconsistently sized
disks into a RAID5 LVM?  Will you still be able to resize the array by
adding and dropping disks?

Just wanting to know.

Thanks,
	Erik.

Re: [linux-lvm] Software raid on top of lvm logical volume

[Theo Van Dinter]

[-- Attachment #1: Type: text/plain, Size: 1370 bytes --]

On Thu, Oct 28, 2004 at 02:54:24PM -0400, Michael T. Babcock wrote:
> >You'd usually want to make your raid devices first, then put LVM on
> >top of it.  I can't really think of any benefits of doing it the other
> >way around.
> 
> I've thought about this numerous times -- there is the distinct resizing 
> advantage.  Namely, if I create a software RAID partition, I can't 
> resize it afterward without destroying it.  I have for example, on 

There's no resizing advantage at all -- if you add another disk to the LVM,
you can just as easily add it to a RAID array instead.

Never mind the fact that if you lose any disk in the LVM, your RAID groups are
dead (unless you split up your LVM disk, but that's what RAID does ...)

LVM gives you flexibility in your disk layout, RAID gives you performance
+ reliability.  LVM builds on RAID's features, but the other way around,
RAID is undercut by LVM.

> I understand this may not be optimal in some situations, but some of us 
> don't have thousands of dollars for SCSI HW RAID controllers.

Neither do I, but this is irrelevent to the conversation.

-- 
Randomly Generated Tagline:
"A bride must be carried over this, but a vampire must be invited into
 it."                                            - Jeopardy Answer
 "What is a Supermarket Spacer, Alex?"           - Theo's Response

[-- Attachment #2: Type: application/pgp-signature, Size: 189 bytes --]

[linux-lvm] LVM DISK DIE

["KieZz"]

Hi
i have a lvm whit 4 disk on a machine whit lvm2 and kernel 2.6.8.1 .
One is dead, i've tried whit pvmove to 2 disk (200 gb -> 120 +80 ) but it die 
before moving 30 Gb.
Now the disk i think is totaly unusable, cfdisk refuse to open it (like 
opening a non exitent device) and i have this:

root@Orange:~# vgscan
  Reading all physical volumes.  This may take a while...
  Couldn't find device with uuid '42cK3V-88Wm-SEMU-1qUb-fAsZ-Fo1V-QFzFvG'.
  Couldn't find all physical volumes for volume group orange_vg.
  Couldn't find device with uuid '42cK3V-88Wm-SEMU-1qUb-fAsZ-Fo1V-QFzFvG'.
  Couldn't find all physical volumes for volume group orange_vg.
  Couldn't find device with uuid '42cK3V-88Wm-SEMU-1qUb-fAsZ-Fo1V-QFzFvG'.
  Couldn't find all physical volumes for volume group orange_vg.
  Couldn't find device with uuid '42cK3V-88Wm-SEMU-1qUb-fAsZ-Fo1V-QFzFvG'.
  Couldn't find all physical volumes for volume group orange_vg.

and also this:

root@Orange:~# pvscan
  Couldn't find device with uuid '42cK3V-88Wm-SEMU-1qUb-fAsZ-Fo1V-QFzFvG'.
  Couldn't find device with uuid '9X3e0u-IqHl-B7Eh-CcGa-7Zbp-Awv8-5dfBnj'.
  Couldn't find device with uuid '42cK3V-88Wm-SEMU-1qUb-fAsZ-Fo1V-QFzFvG'.
  Couldn't find device with uuid '9X3e0u-IqHl-B7Eh-CcGa-7Zbp-Awv8-5dfBnj'.
  Couldn't find device with uuid '42cK3V-88Wm-SEMU-1qUb-fAsZ-Fo1V-QFzFvG'.
  Couldn't find device with uuid '9X3e0u-IqHl-B7Eh-CcGa-7Zbp-Awv8-5dfBnj'.
  Couldn't find device with uuid '42cK3V-88Wm-SEMU-1qUb-fAsZ-Fo1V-QFzFvG'.
  Couldn't find device with uuid '9X3e0u-IqHl-B7Eh-CcGa-7Zbp-Awv8-5dfBnj'.
  PV /dev/hdb         VG orange_vg   lvm2 [189.92 GB / 0    free]
  PV /dev/hdc         VG orange_vg   lvm2 [189.92 GB / 0    free]
  PV unknown device   VG orange_vg   lvm2 [189.92 GB / 0    free]
  PV unknown device   VG orange_vg   lvm2 [189.92 GB / 150.89 GB free]
  PV /dev/hdg         VG orange_vg   lvm2 [114.49 GB / 956.00 MB free]
  PV /dev/hdh         VG orange_vg   lvm2 [74.52 GB / 4.00 MB free]
  Total: 6 [948.70 GB] / in use: 6 [948.70 GB] / in no VG: 0 [0   ]

There is a way to save something ?

Thanks 

Re: [linux-lvm] Software raid on top of lvm logical volume

[Markus Baertschi]

Michael T. Babcock wrote:

>
> I've thought about this numerous times -- there is the distinct 
> resizing advantage.  Namely, if I create a software RAID partition, I 
> can't resize it afterward without destroying it.  I have for example, 
> on occasion, had three disks set up where 1/3 of each was devoted to a 
> RAID-0 very fast striping set for data transfers that had to be fast 
> but if they were lost it wasn't critical, and 2/3 was set up as RAID-5 
> for reliability of another set of data.
>
> I understand this may not be optimal in some situations, but some of 
> us don't have thousands of dollars for SCSI HW RAID controllers.

You can make life easier by creating several raid arrays on the same 
disks and tie them together with LVM.
For example split your three 160GB drives into 4 partitions each and 
create a raid over the three disks.

sda -> sda1+sda2+sda3+sda4, etc.
md0 = sda1+sdb1+sdc1, md1=sda2+sdb2+sdc2, etc.

Markus

-- 
  Markus Baertschi             Phone: ++41 (21) 807 1677
  Bas du Ross� 14b             Fax  : ++41 (21) 807 1678
  CH-1163, Etoy                Email: markus@markus.org
  Switzerland                  Homepage: www.markus.org

[linux-lvm] Re: What is the best way to configure LVM + RAID?

[Peter T. Breuer]

Erik Ohrnberger <erik@echohome.org> wrote:
> OK, so like what is the best way to configure multiple inconsistently sized
> disks into a RAID5 LVM?  Will you still be able to resize the array by
> adding and dropping disks?

Well, you'll just have to make a raid5 of the minimum size (of any three
of your disks :), and do something else with the rest of the remaining
space.

Peter

Re: [linux-lvm] Software raid on top of lvm logical volume

[Scott Serr]

I've done something like Markus explained.  I had this email ready to go 
a couple days ago, but the compose window was hidden for a while... I 
hope it's not too stale for the original poster of this thread:

2.4.26 with latest LVM2 works well.  I did something crazy, alot of 
people probably don't understand why.

I have software RAID5.

md0 = hda5 hdb5 hdc5 hdd5 hde5
md1 = hda6 hdb6 hdc6 hdd6 hde6
md2 = hda7 hdb7 hdc7 hdd7
md3 = hda8 hdb8 hdc8 hdd8

Then I have just one big vgdata and one lvdata.

Why would I want to do this?  I couldn't find any other way to have 
redundancy and be flexible.  If I replace a physical disk with a larger 
one, for instance hde needs to be bigger so I can make md2 and md3 
stretch across that disk too...  then I am able to do pvmove from md2 to 
a scratch disk, recreate md2 across hda7-hde7, then pvmove back.

-Scott

Markus Baertschi wrote:

> Michael T. Babcock wrote:
>
>>
>> I've thought about this numerous times -- there is the distinct 
>> resizing advantage.  Namely, if I create a software RAID partition, I 
>> can't resize it afterward without destroying it.  I have for example, 
>> on occasion, had three disks set up where 1/3 of each was devoted to 
>> a RAID-0 very fast striping set for data transfers that had to be 
>> fast but if they were lost it wasn't critical, and 2/3 was set up as 
>> RAID-5 for reliability of another set of data.
>>
>> I understand this may not be optimal in some situations, but some of 
>> us don't have thousands of dollars for SCSI HW RAID controllers.
>
>
> You can make life easier by creating several raid arrays on the same 
> disks and tie them together with LVM.
> For example split your three 160GB drives into 4 partitions each and 
> create a raid over the three disks.
>
> sda -> sda1+sda2+sda3+sda4, etc.
> md0 = sda1+sdb1+sdc1, md1=sda2+sdb2+sdc2, etc.
>
> Markus
>

Re: [linux-lvm] Software raid on top of lvm logical volume

[Michael T. Babcock]

Markus Baertschi wrote:

> You can make life easier by creating several raid arrays on the same 
> disks and tie them together with LVM.
> For example split your three 160GB drives into 4 partitions each and 
> create a raid over the three disks.
>
> sda -> sda1+sda2+sda3+sda4, etc.
> md0 = sda1+sdb1+sdc1, md1=sda2+sdb2+sdc2, etc.


And how exactly does that allow me to easily change how much RAID-0 or 
RAID-5 storage I have?

If I need more RAID-5 storage and I want to steal it from RAID-0 space, 
I need to dissassemble LVM, then software RAID, then repartition, then 
rebuild RAID, then restore LVM.
-- 
Michael T. Babcock
http://mikebabcock.ca

Re: [linux-lvm] Re: Software raid on top of lvm logical volume

[Michael T. Babcock]

[-- Attachment #1: Type: text/plain, Size: 547 bytes --]

Peter T. Breuer wrote:

>>i do not believe you would have any advantage in stacking md above lvm
>>anyway.
>>    
>>
>
>That is right. In fact it sounds downright risky to me.
>  
>

I've actually considered a few times how one might be able to merge the 
md functionality into LVM2 (device-mapper) for having software-RAID part 
of the logical volume system (since a software-RAID partition is a 
logical volume of another type anyway).

It would require some significant effort, but it seems doable.
-- 
Michael T. Babcock
http://mikebabcock.ca

[-- Attachment #2: Type: text/html, Size: 1040 bytes --]

Re: [linux-lvm] Re: Software raid on top of lvm logical volume

[Erik Ohrnberger]

This solution would seem to be as the holy grail in that it would give you
the best of all worlds: resizable RAID storage.  Or am I wrong about this
assumption?

On Mon, November 1, 2004 11:01, Michael T. Babcock said:
> Peter T. Breuer wrote:
>
>>>i do not believe you would have any advantage in stacking md above lvm
>>>anyway.
>>>
>>>
>>
>>That is right. In fact it sounds downright risky to me.
>>
>>
>
> I've actually considered a few times how one might be able to merge the
> md functionality into LVM2 (device-mapper) for having software-RAID part
> of the logical volume system (since a software-RAID partition is a
> logical volume of another type anyway).
>
> It would require some significant effort, but it seems doable.
> --
> Michael T. Babcock
> http://mikebabcock.ca
> _______________________________________________
> linux-lvm mailing list
> linux-lvm@redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/

[linux-lvm] Re: Software raid on top of lvm logical volume

[Peter T. Breuer]

Michael T. Babcock <mike@mikebabcock.ca> wrote:
> And how exactly does that allow me to easily change how much RAID-0 or 
> RAID-5 storage I have?

It doesn't. 

> 
> If I need more RAID-5 storage and I want to steal it from RAID-0 space, 
> I need to dissassemble LVM, then software RAID, then repartition, then 
> rebuild RAID, then restore LVM.

You have to anyway. You can't change the size of a raid on the fly.

Peter

Re: [linux-lvm] Re: Software raid on top of lvm logical volume

[Clint Byrum]

On Mon, 2004-11-01 at 11:51 -0500, Erik Ohrnberger wrote:
> This solution would seem to be as the holy grail in that it would give you
> the best of all worlds: resizable RAID storage.  Or am I wrong about this
> assumption?
> 

Isn't that what NetApps do already with their "modified RAID4"?

Re: [linux-lvm] Re: Software raid on top of lvm logical volume

[Theo Van Dinter]

[-- Attachment #1: Type: text/plain, Size: 725 bytes --]

On Mon, Nov 01, 2004 at 02:03:19PM -0800, Clint Byrum wrote:
> > This solution would seem to be as the holy grail in that it would give you
> > the best of all worlds: resizable RAID storage.  Or am I wrong about this
> > assumption?
> Isn't that what NetApps do already with their "modified RAID4"?

Most RAID systems let you relayout the disk.  NetApps are just another
example, so is Veritas.

-- 
Randomly Generated Tagline:
quit   When the quit statement is read, the  bc  processor
        is  terminated, regardless of where the quit state-
        ment is found.  For example, "if  (0  ==  1)  quit"
        will cause bc to terminate.
 (Seen in the manpage for "bc". Note the "if" statement's logic)

[-- Attachment #2: Type: application/pgp-signature, Size: 189 bytes --]

Re: [linux-lvm] Re: Software raid on top of lvm logical volume

[Michael T. Babcock]

Erik Ohrnberger wrote:

>This solution would seem to be as the holy grail in that it would give you
>the best of all worlds: resizable RAID storage.  Or am I wrong about this
>assumption?
>  
>

I've been trying to figure out a reason why it wouldn't be doable for a 
few weeks now.

Currently, one can create RAID-0 like performance with LVM2 by 
specifying stripes for one's LVs.  Why not:

lvcreate -L 3GB --raid-level 5 --stripes 3 -n SafeData VG001 /dev/sda 
/dev/sdb /dev/sdc

PV section 1 on sda and sdb are data, section 1 on sdc is XOR data.  PV 
section 2 on sda and sdc are data, and XOR data on section 2 of sdb, and 
so on.

One of the major performance implications would be the larger block 
sizes being dealt with; many people seem to find smaller stripes better 
for performance in RAID-0/5 situations, and by default one would have 
4MB stripes.  With some effort, I'm sure this could be rectified as well.

Thoughts?  Or should I blog about it instead? ;-)

If I didn't have multiple out-of-town server and firewall installations 
this week, I'd code instead :-)
-- 
Michael T. Babcock
http://mikebabcock.ca