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SSD: over-provisioning

Posted: Sat May 24, 2014 2:24 pm
by kyrios

I have read that 10 to 25% of the space of a SSD should be left free or unallocated for over-provisioning.
So for example if you have a PC with a 120Gb SSD, 12 to 25 Gb should always remain free. Many people advise to not allocate this space when partitionning the disk to make sure the space cannot be used.
Initially, I was thinking of using no swap partition at all (there is only a SSD in the computer, no HDD) and with 8Gb of RAM it is not really necessary BUT why not creating a 16Gb swap partition instead of leaving the space unallocated and having no swap ? 99.9% of the time, the swap won't be used so it's +/- as if the space was unallocated, isn't it?

What do you think ?

Re: SSD: over-provisioning

Posted: Sat May 24, 2014 4:54 pm
by austin.texas
I suspect that a swap partition will not be used for over-provisioning. That is a tricky question that you should investigate.
It is true that unallocated space (give it at least 20%) is a bit better than partitioned space.
I have 8GB RAM on a desktop, and I don't think my swap partition has ever been used.

Re: SSD: over-provisioning

Posted: Sat May 24, 2014 5:06 pm
by eanfrid
The main concern for performance is ensuring to keep enough unused capacity. You can do this in 2 ways:
1/ keeping unallocated space on the SSD
2/ oversizing all your partitions to make sure none will ever be filled with e.g. more than 60% of data
The first solution is obviously both easier and maybe smarter.

A swap partition will never get filled unless you have a very serious problem. So, setting up a big swap partition on the SSD is also a smart move (after all, swap space if ever used need to be the fastest you can achieve).

For example, on my smallest SSD (120GB) I chose a mix of oversizing and unallocated space: only 100GB are allocated, in 4 parts: /boot, /, /usr/local and swap (/home is on a RAID1 HDD array). /boot is 1GB, swap is 20GB, / is 50GB and /usr/local uses what remains (~30GB). At the moment each partition is filled under 20% of its capacity and I think the SSD will live forever :) (this SSD is almost 2 years old).

Re: SSD: over-provisioning

Posted: Sun May 25, 2014 7:15 am
by kyrios
It is not necessary to leave free space on each partition of the SSD.
Even if one of the partition if full, as long as there is still enough free/unallocated space on the SSD, the algorythm will manage it fine.

My question is why do most the tutorials recommand to not use a swap when there is enough ram on the computer and no harddisk and to leave 20% to 25% of the space unallocated rather than saying create a swap partition that will most likely never be used (set swapiness to a very low value or even to 0) but the could in some very specific cases be useful and count it as unallocated space ?

Perhaps there is a very good reason for not doing it but I haven't found any info so far...