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Bob

23:13

bwDraco

@UTF-8: Write amplification is typically at its worst with random I/O. Also, 25-35 GB isn't much free space; this space can be readily exhausted (from the standpoint of the SSD controller) during normal use of the SSD. If you can't free up more space, you'll want to do more frequent TRIMs to make sure the controller has spare space available; changing it to daily is a good first step. You may also want to run fstrim manually after write-intensive tasks as appropriate. — bwDraco 5 mins ago

Yikes. 4x write amplification.

If your SSD is low on free space, it's even more critical to frequently TRIM the SSD. The less space you have, the faster it internally runs out of spare space and the higher the write amplification. Running TRIM often helps ensure that the drive has some spare space available.

> To reduce write amplification, most modern systems support a command called TRIM, which tells the drive which blocks no longer contain valid data so they can be erased. This is necessary because the drive would otherwise need to assume that data logically deleted by the operating system is still valid, which hinders the drive's ability to maintain adequate free space.

A: Over-provisioning an SSD - does it still hold?

Modern SSD controllers are smart enough that overprovisioning is not typically necessary for everyday use. However, there are still situations, primarily in datacenter environments, where overprovisioning is recommended. To understand why overprovisioning can be useful, it is necessary to underst...

Bob

huh, we have an Beer, Wine & Spirits?

(pretty sure I've said that word-for-word before, too :P)

djsmiley2k

heh yey

i called it

UPS launching drones from vans

Journeyman Geek

23:28

@Bob or as I call it... Booze.se

bwDraco

Q: Does wear leveling function normally without TRIM?

No matter whether it's about UEFI, phones, or SSDs, Samsung apparently isn't particularly good at implementing standards. Unfortunately, some year ago, I bought a Samsung SSD 840 PRO Series SSD for my laptop which I have been using since (this was before all the info about their non-standard-conf...

DavidPostill

May 24 '16 at 3:24, by Bob

we have a Beer, Wine & Spirits?! o.O

@Bob ^^^

djsmiley2k

Bob

16 mins ago, by Bob

(pretty sure I've said that word-for-word before, too :P)

It doesn't come up that often on HNQ

bwDraco

23:44

@UTF-8, I gave your question some thought and I thought I might have a more detailed discussion. The gist of the issue you're experiencing is that the SSD is internally running out of spare space. SSDs try to spread writes out across whatever free space is available, but if you've written to a particular block before, the SSD's controller will always think that block is in use even if the data is deleted from the filesystem. TRIM tells the SSD the space is no longer in use.

Over time, all the blocks get written over, and random writes can cause data to take up more space on the NAND as the drive spreads out writes. This is because blocks can't be erased directly; see superuser.com/questions/944913/….

UTF-8

yes, and shouldn't the free space the SSD has interally be at least as big as the free space on a trim-supporting FS right after a trim?

bwDraco

@UTF-8 It's complicated, but because the drive can only erase large chunks of data at a time, there tends to be lots of partly-filled blocks.

The SSD will try to erase blocks only as needed to avoid excess wear. Stale pages in each block aren't overwritable; the whole block needs to be erased. Hence, random writes can cause data to take up more space than its actual size.

With TRIM, though, the drive will know that those stale blocks are in fact stale; otherwise, the drive will just treat them as containing valid data. This will cause it to internally run out of spare space faster as it's maintaining stale data.

Long story short, 1 GB of random writes can actually involve more than 1 GB of physical NAND, so they can cause the spare area on the drive to run out faster.

In fact, the drive may actually have less than 30 GB of spare area available even after TRIM because of the way writes are distributed across blocks. Again, SSDs try not to erase and rewrite blocks unless they really need to, so even if your filesystem has 30 GB of free space and you've TRIM'ed the space, there's likely still some stale data on the NAND waiting to be erased at a more appropriate time. The best solution is ultimately to free up more space on the drive.

Maybe someone else can explain this better. @Bob?

Bob

huhwhaza

bwDraco

@Bob Start here: superuser.com/questions/944913/…

Bob

@bwDraco I think you've explained it pretty well.

At the end of the day, it comes down to how much you care about consistent high speeds vs how much space you need.