Unraid 6.12.10: Pre-clear started out very fast, now is running MUCH slower

[RasterEyes]

This isn't a crucial issue for me, just a matter of curiosity.

 

Today I added three new 4 TB SSDs to my array. They were preclearing at a nice clip, almost 450 MB/s, up until about 1 TB was cleared. At that pace the whole job was going to be done in less than three hours.

Then things slowed down quite a bit, however, generally clearing at a speed from 50-100 MB/s. I was about to say the current speed was 85, but it dropped to less than 30 sometime after I started typing this post, and has since climbed back to around 65. I'm about 42% done now, with the wildly-fluctuating completion time estimated at around 11 hours from now.

The array isn't busy doing anything else. It's idle apart from the preclear operation.

I did just notice that my version of Unassigned Devices Preclear is 2023.05.20, with an update to 2024.04.23a available. Maybe that has something to do with the big speed drop? I have no intention of updating that plugin while preclear is already running, however.

Could this just be the normal behavior of the SSD, maybe a response to the SSD warming up from running so steadily for a while?

Edited June 16 by RasterEyes
Period -> comma

[JorgeB]

You should not preclear flash based devices, and it's normal for writes to start much faster, while they use a pseudo SLC cache, and then it slows down.

[RasterEyes]

8 hours ago, JorgeB said:

You should not preclear flash based devices, and it's normal for writes to start much faster, while they use a pseudo SLC cache, and then it slows down.

It wasn't my decision. I plugged in the new drives, started up, and that process started automatically.

As for "it's normal for writes to start much faster", the cache isn't 1 TB in size. I could see cache being an issue for a few MB or GB, but it sure as hell wasn't cache that made the initial writes run at 450 MB/sec for the first whole TB.

[JonathanM]

7 minutes ago, RasterEyes said:

It wasn't my decision. I plugged in the new drives, started up, and that process started automatically.

Preclear is always optional, it's an addon that is used to zero and test a drive before it's put into a parity protected array.

 

IF the drive being added to a parity protected array slot does NOT have a valid preclear signature, then the normal operation of clearing the drive by writing all zeros is started, and the drive is not available for formatting and use until that clearing has completed, so adding the drive doesn't invalidate parity.

 

Preclear was developed to allow you to do that step BEFORE adding the drive to the array, so it could be formatted and used immediately.

 

What JorgeB was saying, is that using the preclear utility on a solid state drive is not recommended for several reasons. If you are using SSD's in the parity protected array, clearing is still needed, but preclear isn't.

 

Using SSD's in the parity protected array can reduce their performance, TRIM is not possible, and write speed will be limited by the parity drive.

[Kilrah]

2 hours ago, RasterEyes said:

the cache isn't 1 TB in size

It could be, or the drive not being ran at full speed could extend the time it takes to fill. Check reviews of that drive model for sustained write performance.

 

 

Edited June 17 by Kilrah

[RasterEyes]

3 hours ago, JonathanM said:

F the drive being added to a parity protected array slot does NOT have a valid preclear signature, then the normal operation of clearing the drive by writing all zeros is started, and the drive is not available for formatting and use until that clearing has completed, so adding the drive doesn't invalidate parity.

Perhaps I'm just using the wrong terminology. The array was writing (presumably zeros) two all three new drives, whether that's called preclearing or not. It's all done now, everything is fine. I was just curious about the sudden drop in speed.

[JorgeB]

12 hours ago, RasterEyes said:

the cache isn't 1 TB in size.

Most devices use a pseudo cache, it will vary with how much space is used, if the device was empty it's very possible that it could be around 1TB.

[JorgeB]

8 hours ago, RasterEyes said:

The array was writing (presumably zeros) two all three new drives, whether that's called preclearing or not.

If you added the devices as new arrays disks they will be cleared first, with flash media it may be worth to do a new config and re-sync parity instead, top avoid the write cycle, though in the end, probably not going to add much.