[Bglug] [kwlug-disc] Installing onto an SSD

Anthony Morassutti moralater9 at gmail.com
Thu Mar 11 12:38:55 EST 2021

Can I share this message with a distro maintainer?

On 11/03/2021, Remi Gauvin <remi at georgianit.com> wrote:
> On 2021-03-11 10:56 a.m., Anthony Morassutti wrote:
>> What is a "trim" command??
> Unlike magnetic disks, Flash memory cells can not be overwritten on the
> spot.  When a 4KB data block on an SSD is changed, the drive actually
> copies the entire 512KB (I'm not sure if this is the real value, but
> will use it here for illustration) block to a new cell.  The old Cell is
> erased entirely for it to be re-used.
> On traditional computer magnetic media, when a file is deleted, the data
> blocks are not actually erased.  Eventually, the filesystem will simply
> re-use that space if it needs it.
> This causes 2 problems for SSD's.  In time, all the blocks on the SSD
> will have Data in them, even if the filesystem considers it all empty
> space.  In terms of performance, the more free blocks available, the
> faster the SSD will be able to write data.  SSD write speed will
> decrease dramatically once the drive is more than 50% full.  The 2nd
> problem, since the drive is constantly moving that data to fresh cells
> as part of it's wear leveling, copying stale data around will increase
> wear unnecessarily.
> The traditional method of really erasing data on a HDD (by replacing it
> with 0's) will not alleviate these issues.  As far as the SSD is
> concerned, all 0's is still data.
> Very early in the transition to Flash media for computer storage, (ie,
> SSD drives), the TRIM (or Discard) command  was added to the ATA
> specification.  This is a special command that the Operatring system
> sends to the SSD to tell it 'this data block is empty.'  From that point
> forward, the SSD is no longer to preserve that data.
> Linux has 3 ways to leverage this ability.
> 1. You can mount your filesystem with the 'discard' option in fstab.
> This will instruct your filesystem to issue a TRIM command on the data
> blocks whenever a file is deleted.  It was found, however, this can hurt
> performance when the workload includes lots of file deletions.
> 2. There is an fstrim command.  Fstrim will instruct your filesystem to
> TRIM all the unused space in one bulk request.  Most distros put this in
> a cron.daily or cron.weekly job.
> 3.  *DANGEROUS*.  Linux has a blkdiscard command that will TRIM an
> entire partition or disk.  This is a great command to use to blank a
> drive for re-use, but note: it is not considered sufficient in the case
> where a secure erase is required.
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