"keep directories in sync" or "raid alternatives"

Jogie214

Hi,

I used a raid1/LVM array for quite some time, and I'm happy with the setup. Now I got my hands on a nice SSD, and I would like to migrate my /usr directory to the SSD, BUT I also would like to keep an up-to-date copy of /usr on the raid array.
Now the main question, what's the best approach to this? I know rsync can keep local directories in sync, I also stumbled upon unison, but I'm not certain if there isn't a more elegant solution.
So any ideas/suggestions?

Thanks,
Sebastian
_________________
Desktop: Core 2 Duo E6600 / Asus P5B / 4GB DDR2-800 / Ati HD-4850/ Sidux amd64
Server : Phenom2 905e / Asus M4A785TD-V / 8GB DDR3-1333 / Geforce 8600GT / Gentoo amd64
Laptop : Core 2 Duo 5500 / HP nx7300 / 4 GB-RAM / Intel / Gentoo amd64

gorkypl · Guru Joined: 04 Oct 2010 Posts: 440 Location: Kraków, PL

For periodic copies of whole directory running rsync from cron is fine, but you will probably need to force it also on shutdown, to prevent partial syncs.
You can also just add SSD to RAID1 array and set it as a preferred read/write drive (HDDs will be sync'd in background).
_________________
BTW, TWM FTW!

Jogie214 · Posted: Tue Apr 10, 2012 2:02 pm Post subject:

Hi,

and thanks for your answer.
I've read that SSD's are not really the best choice for a raid using linux, since the raid array would leave no empty blocks on the device, also some metadata would wear down the cells rather fast. I haven't checked whether it's the truth, since the drive is too small, both HDDs in the raid array are 1GB in size (the SDD has "only" 64GB), the space inside the array is split into different volume groups. I'm not sure whether it would be possible or a good idea to build a raid from a LVM volume inside a raid array with an SDD.
I was hoping for some kind of monitoring daemon (woken by e.g. inotify), that syncs the device as needed.

Greets,
Sebastian
_________________
Desktop: Core 2 Duo E6600 / Asus P5B / 4GB DDR2-800 / Ati HD-4850/ Sidux amd64
Server : Phenom2 905e / Asus M4A785TD-V / 8GB DDR3-1333 / Geforce 8600GT / Gentoo amd64
Laptop : Core 2 Duo 5500 / HP nx7300 / 4 GB-RAM / Intel / Gentoo amd64

gorkypl · Guru Joined: 04 Oct 2010 Posts: 440 Location: Kraków, PL

Jogie214 · Posted: Tue Apr 10, 2012 6:54 pm Post subject:

Hi,

thanks for pointing out that the trim issue is fixed, I'll check if there is a kernel version around with this patch applied.
LVM mirroring (I've to admit, I forgot about it) sound also very promising, but as far as I know, I need three devices for that, but I guess I'll do a few google searches and be wiser afterwards.
As soon as I've found something defined, I'll let you know.

Greets,
Sebastian
_________________
Desktop: Core 2 Duo E6600 / Asus P5B / 4GB DDR2-800 / Ati HD-4850/ Sidux amd64
Server : Phenom2 905e / Asus M4A785TD-V / 8GB DDR3-1333 / Geforce 8600GT / Gentoo amd64
Laptop : Core 2 Duo 5500 / HP nx7300 / 4 GB-RAM / Intel / Gentoo amd64

djdunn · Posted: Wed Apr 11, 2012 6:53 am Post subject:

if you don't want to make it raid why not just use Tar or some other backup software, many are quite eloquent

and you need 3 physical volumes for lvm mirroring one for the data one for the mirror and one for the log, there's a way you can use less physical volumes with mirror core log, however every time you reboot you have to rebuild the mirror cause there's no log.
_________________
Now, with penguins, (cuddly such), "contented" means it has either just gotten laid, or it's stuffed on herring. Take it from me, I'm an expert on penguins, those are really the only two options.

--Linus Torvalds

Jogie214 · Posted: Thu May 03, 2012 4:41 pm Post subject:

Hi,

I promised to give some feedback as soon as I've implemented some form of sync option. It's not automatic, but it works quite flawlessly so far, I use a cron job to run the script every 24h.
I decided to give rsync a try, the script I use is enclosed in this post see below. LVM mirror is no option, since I don't have a third device.
Everything is running smooth so far.