On 30/10/2012 15:29, Curtis J Blank wrote:
On 10/30/12 04:49, David Brown wrote:
On 28/10/2012 19:59, Curtis J Blank wrote:
I've got two new SSD's that I want to set up as RAID1 and use strictly
for the OS and MySQL DB's partitioned accordingly.
I'll be using the 3.4.6 kernel for now in openSuSE 12.2 with ext4. So
after a lot of Google'n and reading it is my understanding that discard
is not sent to the devices via the raid drivers. I am aware of Shaohua
Li's patches to make it work but am not inclined to use them due to
openSuSE's Online Update replacing the kernel. I'm not against patching
and gen'ing a kernel, that used to be SOP, but just don't want deal with
that overhead. Of course unless I really need to.
So I've read, and if I understand things correctly, I can use LVM and
RAID1 and the the discard commands will be sent to the devices. Is that
correct and currently the only way or is/are there other ways?
I've also read that a lot of people are saying TRIM isn't needed because
the SSD's garbage collection is so good now TRIM isn't needed. But I
don't see how that could work because the SSD's don't have access to the
file system so they don't know which pages in the blocks are marked
unused to do any consolidation and erasing. And using TRIM is suggested
in a OCZ document I read and who's drives these are. Unless, the SDD
when it has to change a page moves the whole block then erases the old
block? But without TRIM in could be moving invalid data too because it
doesn't know that and that to me sure doesn't sound efficient and this
operation would be a perfect time to get rid of the invalid data if it
did know.
TRIM is not necessary.
In some situations, TRIM can improve speed - in other cases, it can make
the system significantly slower. And it is only ever a help until the
disk is getting fairly full.
Before deciding about TRIM, it is important to understand what it does,
and how it works. TRIM lets the filesystem tell the SSD that a
particular logical disk block is no longer in use. The SSD can then
find the physical flash block associated with that logical block, and
mark it for garbage collection.
If TRIM had been specified /properly/ for SATA (as it is for SCSI/SAS),
then it would have been quite useful. But it has two huge failings -
there is no specification as to what the host will get if it tries to
read the trimmed logical block (this is what makes it terrible for RAID
systems), and it causes a pipeline flush and stall (which is what makes
TRIM so slow). The pipeline flushing and stalling will cause particular
problems if you have a lot of metadata changes or small reads and writes
in parallel - the sort of accesses you get with database servers. So
enabling TRIM will make databases significantly slower.
And what do you lose if you /don't/ enable TRIM? When a filesystem
deletes a file, it knows the logical blocks are free, but the SSD keeps
them around. When the filesystem re-uses them for new data, the SSD
then knows that the old physical blocks can be garbage-collected and
re-used. So all you are really doing by not using TRIM is delaying the
collection of unneeded blocks. As long as the SSD has plenty of spare
blocks (and this is one of the reasons why any half-decent SSD has
over-provisioning), TRIM gains you nothing at all here. (If you have a
very old SSD, or a very small one, or a very cheap one, then you will
have poor over-provisioning and poor garbage collection - TRIM might
then improve the SSD speed as long as the disk is mostly empty.)
It is possible that blocks that could have been TRIMMED will get
unnecessarily copied as part of a wear-levelling pass - but the effect
of this is going to be completely negligible on the SSD's lifetime.
So TRIM complicates RAID, limits your flexibility for how to set up your
disks and arrays, and slows down your metadata transactions and small
accesses.
TRIM /did/ have a useful role for early SSDs - in particular, it
improved the artificial benchmarks used by testers and reviewers. So it
has ended up being seen as a "must have" feature for both the SSD
itself, and the software and filesystems accessing them.
Thanks for the explanation, makes a lot of sense, has me leaning towards
not using TRIM.
But your explanation focused on blocks, leaving out pages. Does TRIM
info sent to the device only do that on the block level or does it do it
at the page level? I was thinking that if it did it at the page level
the SSD's garbage collection would consolidate blocks by removing unused
pages (akin to defragmenting) then erasing those pages thus making them
ready to be written.
I was not using "block" in a particularly strict of formal way. There
are a number of different levels of structure involved here, including
"logical blocks", "sectors", "allocation units", "erase blocks", "write
pages", etc. I am simply talking about "lumps of data", rather than any
specific structure.
As far as the computer is concerned, it deals with "sector numbers" of
512 byte or 4K sectors. It is up to the SSD to map these logical
numbers to physical pages within flash erase blocks. The PC has no way
of knowing whether a given set of logical sectors are mapped to pages
within the same erase block or different ones.
You are right that the SSD's garbage collection routines will sometimes
collect together the used pages of an erase block, and copy them over to
another erase block, so that the first erase block can be recycled. But
this is done independently of the TRIM, and is part of the normal
garbage collection function.
mvh.,
David
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