Re: How to Boot with LVM
On Tuesday, September 8, 2015 at 3:50:04 AM UTC-5, Rick Thomas wrote:
> Hi Ray,
>
> I'll try to answer your questions...
>
> On Sep 7, 2015, at 4:36 PM, ray wrote:
>
> > Rick,
> >
> > Thank you for responding and providing all the info.
> >
> > On Monday, September 7, 2015 at 6:20:07 AM UTC-5, Rick Thomas wrote:
> >> On Sep 5, 2015, at 7:24 PM, ray wrote:
> >>
<snip>
> > I have 3 pairs of SSDs, each pair in a RAID0.
>
> I would use RAID1 on each pair (3 SSDs worth of "usable" space), or RAID5 or 6 on a larger aggregate. E.g. RAID6 on all 6 drives (gives 4 SSDs worth of "usable" space), or RAID5 on 6 drives (gives 5 SSDs worth of "usable" space). Each of those configurations can survive a loss of one SSD (or two, in the case of RAID6) without data loss.
>
> Your choice of RAID0 in pairs gives the full 6 SSDs worth of "usable" space, but has zero redundancy. If that works for you, that's great.
>
> I've got enough experience (40 years) as a sysadmin to have seen users tearing hair over lost data (I always had backups -- often tape in those days -- so the only thing really lost was uptime, but you get the point...)
( When I bought a tape real for the IBM 360, I held onto my boxes of cards for 4 months. I only had to use them once when I overwrote my tape with an empty library. I keep the tape for 5 years until I was sure there would be nothing to that could read it. My new storage was 8" floppies.)
>
> In case it's not clear, "usable" means space left over after subtracting out all the redundant data in the array.
>
> >
> >>
> >> I configure a small (<1GB) "/boot" partition as a primary partition (e.g. /dev/sda1) on one of the disks, with the same space on the other disk unused. [1] I make another primary partition (e.g. /dev/sd[ab]2), on each of the disks, sized to be one half of the size I want for my swap. The rest of the space on each disk goes into a single, large, logical partition (e.g. /dev/sd[ab]5).
Yes, I have made space on other disks for this also. I don't know how to use them but it seemed the redundancy may come in handy.
> >>
> > This is similar to my setup. I have the swaps on a separate RAID0.
>
> Should work fine. Of course you should read Pascal's post for a different point of view.
Yes, those consideration are ringing in my head. It will be a trade-off consideration from now on.
>
> >
> >> The two swap partitions I set up as a RAID0 (e.g. /dev/md0). This will be my system swap. [2]
> >>
> >> The two large logical partitions, I configure into a RAID1 (e.g. /dev/md1). [3]
> >>
> > I made my large partition a primary. Could this be problematic?
>
> It would only be a problem if you need another primary partition for something. You only get 3 primary partitions on a disk, so I like to leave one free "just incase".
>
> >
<snip>
> >>
> > I would like to know more about the purpose of these criteria - please.
>
> Putting /tmp on tmpfs is for speed. If your /tmp usage fits in RAM (after allowing for apps, data, and disk buffering) you get RAM speed access to your temporary data. If it overflows, then the excess goes into swap, so you're no worse off than if you had /tmp on hard disk in the first place. This can make large compilations (as an example) run *much* faster. On the other hand, if you are tight on RAM, putting /tmp on disk doesn't hurt, and eliminates a source of contention for RAM which is assumed to be a scarce resource.
This sounds like an experiment I will need to exercize.
>
> The 50% figure is just a rule of thumb I picked up over the years. There's nothing magic about it. And, as always, YMMV. It's highly application dependent.
>
> >
<snip>
>
> It might make sense to create a logical volume with some of the unused space in the volume group called (for lack of a better name) /backup. It's "reliable" in the sense that it resides on a RAID1/5/6 array, so it's a good place to put your backups of things like /boot. For "ultimate" backup, I usually use huge (e.g. 4TB or larger) external USB disk drives. I don't RAID them, instead I have two or three and rotate amongst them using each one for a week, then swapping it out for the next one. The currently offline drive(s) I keep in a fire-proof safe, preferably in a separate building from the server...
>
Thanks,
Ray
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