[Discuss] ssd's in linux

[feenberg at nber.org (Daniel Feenberg)]

On Fri, 8 Nov 2013, Chuck Anderson wrote:

> On Fri, Nov 08, 2013 at 06:15:32AM -0500, Stephen Adler wrote:
>> I'm thinking of upgrading my linux system by adding an SSD drive to
>> use as my system disk. Has anyone done this? Any pros and cons
>> regarings using SSD's? I'm more intrested in the cons. I'm worried
>> about the life expectancy since there is a limited number of writes
>> one can do to the NAND memory. Do I need to setup a raid array to
>> ensure the SSD contents are correct?
>
> After trying out various methods of combining a SSD with a spinning
> HDD (write-behind RAID etc.) I've settled on keeping the two drives
> separate for performance reasons, and then using Lsyncd to synchronize
> files between them in almost-realtime as an online backup.  Lsyncd
> uses the inotify Linux kernel feature to determine what files/dirs
> have changed in real time, and then it spawns rsync to do the actual
> copies.
>
> I partitioned the SSD and HDD identically, except for some extra space
> on the SSD where I put a swap partition (yes I'm swapping to SSD but I
> don't dip into swap much).  I also made sure to install the bootloader
> on the HDD as well so I can boot from the HDD if the SSD dies.  Not
> quite as resilient as RAID, but good enough for a desktop.
>
> Just be sure to align your partitions (and any other block layers such
> as LVM, LUKS, filesystems, etc.) to 2MiB boundaries for best SSD
> performance and wear.  This should be the default for all modern
> tools/distros these days.  You may also want to enable TRIM on the
> SSD, although recent research brings into question the value of TRIM
> and whether it actually hurts performance rather than helping it.
>
> I'm just not going to be paranoid about wearing out the SSD,
> especially since I have lsync there to keep my backups.  With the
> whole system on SSD performance is outstanding.

Do we know the form of SSD failures? That is, if the failures are of the 
form of unsignaled write failures, then your lsyncd will copy the bad bits 
to backup, defeating your purpose. Only if the failures are bits rot 
sometime after being correctly written, will your backup be helpful. I 
have looked, but never found, information on this point.

dan feenberg
feenberg at nber.org