> On Feb 1, 2022, at 6:00 PM, Warner Losh via cctalk <cctalk@classiccmp.org>
> wrote:
>
> On Tue, Feb 1, 2022 at 12:42 PM Grant Taylor via cctalk <
> cctalk@classiccmp.org> wrote:
>
>> On 2/1/22 2:14 AM, Joshua Rice via cctalk wrote:
>>> There's several advantages to doing it that way, including balancing
>>> wear on a disk (especially today, with SSDs), as a dedicated swap
>>> partition could put undue wear on certain areas of disk.
>>
>> I thought avoiding this very problem was the purpose of the wear
>> leveling functions in SSD controllers.
>
> All modern SSD's firmware that I'm aware of decouple the physical location
> from the LBA. They implement some variation of 'append store log' that
> abstracts out the LBAs from the chips the data is stored in. One big reason
> for this is so that one worn out 'erase block' doesn't cause a hole in the
> LBA
> range the drive can store data on. You expect to retire hundreds or
> thousands of erase blocks in today's NAND over the life of the drive, and
> coupling LBAs to a physical location makes that impossible.
Another reason is that the flash memory write block size is larger than the
sector size exposed to the host, and the erase block size is much larger than
the write block size. So the firmware has to keep track of retired data, move
stuff around to collect an erase block worth of that, then erase it to make it
available again to receive incoming writes.
The spare capacity of an SSD can be pretty substantial. I remember one some
years ago that had a bug which, in a subtle way, exposed the internal structure
of the device. It turned out the exposed capacity was 49/64th of the physical
flash space. Strange fraction, I don't think we were ever told why, but the
supplier did confirm we analyzed it correctly.
paul
