​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@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.

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