LVM-Cache on fresh Ubuntu server install

[Stephen Partington]

​The biggest issue i have is that i am working with a mac mini. the 1T
drive is a 5400 rpm drive. this has been my home server for a while running
server 2016, but it is time to put something more useful on it.

Part of this is specifically to learn how to use lvm and lvmcache.​ And ram
is on the list for this server just not in the budget yet.

I am not worried about catastrophic failure or the like as nothing lives on
this server that does not already live anywhere else.

I did the install with the Ubuntu server install so i could at least get
lvm set up partly ahead of time. so it looks like I am in a good place. I
was reading that article as i received your email.

So now to extend the vg create the cache and meta lv's and turn things on.

lvm> pvscan
  PV /dev/sdb2   VG System          lvm2 [922.20 GiB / 0    free]
  PV /dev/sda3                      lvm2 [117.38 GiB]
  Total: 2 [1.02 TiB] / in use: 1 [922.20 GiB] / in no VG: 1 [117.38 GiB]
lvm> pvs
  PV         VG     Fmt  Attr PSize   PFree
  /dev/sda3         lvm2 ---  117.38g 117.38g
  /dev/sdb2  System lvm2 a--  922.20g      0
lvm> vgs
  VG     #PV #LV #SN Attr   VSize   VFree
  System   1   1   0 wz--n- 922.20g    0
lvm> vgdisplay
  --- Volume group ---
  VG Name               System
  System ID
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  2
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                1
  Open LV               1
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               922.20 GiB
  PE Size               4.00 MiB
  Total PE              236082
  Alloc PE / Size       236082 / 922.20 GiB
  Free  PE / Size       0 / 0


On Fri, Jul 8, 2016 at 11:02 AM, Matt Graham <mhgraham at crow202.org> wrote:

> On 2016-07-08 10:00, Stephen Partington wrote:
>
>> 1T spinning disk and 128G SSD installed. [...] going with LVM-Cache
>> Sadly the Ubuntu server installation does not have the option of
>> setting up LVM for anything other than its most basic configuration
>>
>
> LVM is complicated, and using LVM-cache is a bit more complicated than
> just making some PVs and putting them into a VG and then making LVs.
>
> What I understand about LVM is that you can build your LVM group and
>> then migrate/expand it without a full install/wipe or 3rd disk device
>> to host your core os installation. Can this be done for LVM cache as
>> well?
>>
>
> Yes.  Your setup would be something like this:  2 PVs, one on the fast
> disk, one on the slow disk[0].  One VG with both PVs in it.  Multiple LVs.
> One named root on the fast disk (mounted at / obviously), one small one
> named cache-meta on the fast disk, one larger one named cache on the fast
> disk, and finally one larger one named origin on the slow disk.  You'd
> probably mount the last one on /data or /home or wherever makes sense to
> put the large amount of stuff you want to serve up.  I suppose you could
> put / on the spinny-disk and use almost all of the SSD for cache too.  Your
> call.
>
> If you already have some PVs and LVs set up, you might need to use pvmove
> and lvresize and resize2fs to resize filesystems and LVs and move them to
> the appropriate PV.  These things are of course doable from a running
> system and should not cause any problems.
>
> https://rwmj.wordpress.com/2014/05/22/using-lvms-new-cache-feature/
> describes all the commands you'd need to run to make this stuff happen.
> And getting an additional 4G or 8G of RAM might help more than using an SSD
> as a cache, depending on what exactly is going on.
>
> (I haven't actually tried this, but I've got a 32G unused partition on my
> SSD right now, and this could be a fun weekend project.  I will let you
> know if I run into any problems.)
>
> [0] With 1T disks as cheap as they are, consider a softRAID-1 instead of a
> single spinny disk.  Disk failures are No Fun At All.
>
> --
> Crow202 Blog: http://crow202.org/wordpress
> There is no Darkness in Eternity
> But only Light too dim for us to see.
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-- 
A mouse trap, placed on top of your alarm clock, will prevent you from
rolling over and going back to sleep after you hit the snooze button.

Stephen
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