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ScanDisk Extreme Pro SSD - Use NCQ trim?
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dman777
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PostPosted: Tue Jun 28, 2016 8:36 pm    Post subject: ScanDisk Extreme Pro SSD - Use NCQ trim? Reply with quote

Does the ScanDisk Extreme Pro use NCQ trim? I found this old article and I am inclined to think they do http://kb.sandisk.com/app/answers/detail/a_id/8146/~/native-command-queuing-(ncq)-defined.

What I do not understand is how come this was not blacklisted in the linux kernel for it like the Samsung Pro drives https://github.com/torvalds/linux/blob/e64f638483a21105c7ce330d543fa1f1c35b5bc7/drivers/ata/libata-core.c#L4238
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NeddySeagoon
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PostPosted: Tue Jun 28, 2016 10:45 pm    Post subject: Reply with quote

dman777,

Trim is a background operation. NCQ Trim is not useful as the there is no hurry for the trim to complete and no time to be saved.
The trim command may go in the NCQ queue but the drive will only make a note of it.
When some space it trimmed, its up to the drive when it actually does the erase.
There are advantages and disadvantages in waiting, depending on how full the device is (when will the space be needed) to trimming too often and incurring extra writes as a result.

The drives erase block size is much larger than the write block size, so any data in the block about to be erased that isn't being trimmed, needs to be moved.
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dman777
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PostPosted: Wed Jun 29, 2016 3:04 am    Post subject: Reply with quote

Hmm.... this contradicts what I read in the wikipedia, so I am probably misunderstanding....

https://en.wikipedia.org/wiki/Trim_(computing)#Shortcomings

Quote:
The non-queued nature of the command requires the driver to first wait for all outstanding commands to be finished, issue the TRIM command, then resume normal commands. ...This penalty can be minimized in solutions that periodically do a batched TRIM, rather than trimming upon every file deletion, by scheduling such batch jobs for times when system utilization is minimal.

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NeddySeagoon
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PostPosted: Wed Jun 29, 2016 4:24 pm    Post subject: Reply with quote

dman777,

I know of one user who was able to recover data from a deleted file, several days after it was trimmed.
This is anecdotal evidence of one user and one drive.
This particular drive did not appear to be in a hurry to perform the erase operation that it was authorised to do.
Drives will vary.

Performing the erase as soon as its indicated leads to as increased number of writes, that shorten drive life.
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