On 07.11.2017 03:01, Michael Stone
wrote:
On
Tue, Nov 07, 2017 at 02:20:56AM +0500, Alexander V. Makartsev
wrote:
As I know SSDs are different in a way HDDs
write their data. HDDs write
directly to the free block and are done. SSDs on the other hand
will have to
erase free block first to get it ready for writing and then
write to it. This
is because how NAND work. It's even worse, if a destination
block has some data
on it already and there is some data needs to be written onto
it, then SSD will
copy data already written, then it will fully erase destination
block, then it
will concatenate copied data with data to be written and finally
will write
merged data to the block.
This is mostly correct. The erasing is done in "erase block" size
units (much larger than disk blocks), while writing is done in
"pages", usually with hundreds of pages per erase block. So a
4kbyte write might trigger a multi-megabyte erase if there aren't
erased pages available for writing. If an erase block has been
partially written, a write can go to an unwritten page without any
additional steps. In practice, most drive writes go immediately to
already-erased blocks, with the garbage collector erasing blocks
while the drive is idle (rather than actually reading, erasing,
and writing for every write). The drive then keeps track of which
physical address is associated with which logical address--that
relationship changes as blocks are written and rewritten, and it
is not the case that the data on the drive is physically ordered
the same way it appears logically.
TRIM will erase blocks marked as deleted
beforehand even if they are not needed
for writing right now. This overhead is the reason that causes
increased drive
wear and decrease it's lifespan.
No, TRIM does not cause extra erases that wear out a drive. If you
don't TRIM blocks and mark them as unused, the garbage collector
will still erase blocks it doesn't need yet in order to have pages
available for future writes. (Either after the garbage collector
decides to launch a read & rewrite cycle to consolidate free
space, or when all the pages
in the erase block have been rewritten to a new erase block and
the old erase block is known to be empty) The only differences
introduced by using TRIM is that the drive can skip the possible
read & rewrite step, and the drive can increase the free pool
so more new pages can be written before a garbage collection is
forced.
I still think there are scenarios where NAND ICs will suffer extra
writes, when TRIM is constantly used.
Also, can you explain what logic is behind this quote from "man 8
fstrim":
"Running fstrim frequently, or even using mount -o discard,
might negatively
affect the lifetime of poor-quality SSD devices."
I think I will find my answers in this huge article on wikipedia
when I wade through it eventually, should be quite interesting read:
https://en.wikipedia.org/wiki/Write_amplification
--
With kindest regards, Alexander.
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