SSD questions

Matthew Dillon dillon at
Thu Mar 26 15:49:51 PDT 2015

:I just got a 1TB 850 EVO and I want to replace my old Intel 40GB SSD boot
:drive. I was thinking I'd just swap the drives and use the DF Installer to
:set it up. It's been more than a few years since I've used the installer,
:so questions:
:- Does it support setting up TRIM on an SSD drive?
:- Is there anything I have to do post-install for the SSD?

    TRIM can be used to effectively erase the device prior to initial
    partitioning.  I usually do it when I fdisk the drive (fdisk -IBE).
    Not sure if the installer will use trim, I don't think it does.

    HAMMER itself will not use TRIM, and I do that on purpose.  HAMMER
    (and HAMMER2) use a relatively large block size which has the
    effect of keeping the SSDs write combining tables relatively clean.

    Generally speaking I don't like TRIM.  It has some severe determinism
    issues and can trigger bugs in SSD firmwares (less so now, probably,
    but still hard to trust).  A better solution is to simply reserve 
    and leave unused 5% to 10% of space at the end of the partition, which
    the SSD will add to its own internal reserve.  That combination should
    be enough to get most of the benefit of TRIM without having to use TRIM,
    and is more deterministic in terms of wear (not necessarily better, just
    more deterministic... easier to estimate).

:=E2=80=8BThe current SSD is partitioned as follows:
:a: 700MB boot
:b: 32GB encrypted swapcache
:d: 2GB root

    You don't need to run with swapcache enabled if your entire storage is
    on the SSD, but it is always good to have a large amount of normal swap
    for things like /tmp.

    What I recommend is:

    a: 1GB boot
    b: 32GB swap
    d: * hammer		(root and everything else on one hammer filesystem)

:I've configured initrd to transfer to the real encrypted root on a disk
:drive. For the new setup, I'd like that encrypted root to be on the SSD. At
:the time I built this machine, it was recommended that some unpartitioned
:space be left on the SSD to allow for a pool of replacement blocks. Is this
:still necessary with modern SSDs?
:Any tips on setting this up would be greatly appreciated.

    Yah, see above.  Leaving 5%-10% unpartitioned space on a SSD is almost
    as good as using TRIM, and in my view is more deterministic.

    I would not use the installer to try to set it up.  I'd do it manually
    if you know enough about how installs work to do that.  Leave your 40G
    SSD in place until you have your new SSD fully operational and booting.
    Be careful not to accidently wipe your original SSD while messing with
    the new one :-)


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