Ted Unangst wrote:
On Thu, Nov 5, 2009 at 3:32 PM, Joachim Schipper
<joac...@joachimschipper.nl> wrote:
Still, the point I was trying to make - that leaving part of your disk
unpartitioned doesn't really help - stands, no?
Depends. Partitioning or not partitioning is unlikely to make a
difference, as it's writes that matter. However, if you need 32G of
space, but buy a 128G disk, that will last longer. The 32G will
almost certainly last "long enough", but if you're paranoid, you can
buy assurance by getting a larger disk.
So leaving part of the disk unpartitioned implies you have extra
space, which does help, but it's not the partitioning itself which
makes the difference.
I agree with Ted and would like to expand on it:
Partitioning the disk to extend its life only works after a secure erase
of the drive and never ever touching the non-partitioned space.
The reason for this is due to the SSD's internal mapping of logical
OS-addressable sectors to its physical sectors.
Take for example a fresh (newly bought or secure erased) 40G drive and
put a 32G partition on it. For the SSD controller there will always be
8G worth of sectors that have never been touched and therefore can be
used without wear-leveling its contents first. But if you even once use
this partition and fill up the 8G, the mapping table will be filled up
as well and even after deleting the content and/or the partition it will
from this point on shuffle around these obsolete 8G of data. No type of
formatting can free up these sectors other than secure erasing, which of
course only applies to the whole drive. To be clear, something like a
zero-format (dd if=/dev/zero) doesn't help as well, as the SSD
controller would then shuffle around sectors with only 0x00 in it.
So it takes self-control on your part to keep the SSD long-lived and fast.
Maybe some better way would be to use the Host Protected Area feature.
It limits the amount of OS-addressable sectors, thus the disk space
always appears smaller to the OS than it really is and you can use the
"whole" disk. In fact I suspect this is what Intel does with its X25-E
SLC drives. The amount of sectors of the 32G drive is 62,500,000 which
is an "odd" number since disks usually don't have such a nice looking
number of sectors. Unfortunately the price point keeps me from verifying
that. The X25-M consumer drives don't have a HPA-hidden space.
I would therefore be thankful if someone with such a precious drive
could check on that. I checked my X25-M with HDAT2 which boots into MS-DOS.
For the sake of completeness: All of this applies only to SSDs without
the TRIM ATA command. TRIM-enabled SSDs and OSs don't need this workaround.
