[SOLVED] Which ashift should i use for my ZFS pool?


May 1, 2022
Hello all,

I had some Samsung 980 Pro NVME SSDs left and wanted to put them into a ZFS pool. Now I was wondering which ashift value I should put in...

I read that ashift=12 is good for 4k sector size, well yes but somehow Linux only shows me a logical and physical sector size of 512 on my SSDs:

root@pve:~# fdisk -l /dev/nvmen1 fdisk: cannot open /dev/nvmen1: No such file or directory root@pve:~# fdisk -l /dev/nvme1n1 Disk /dev/nvme1n1: 931.51 GiB, 1000204886016 bytes, 1953525168 sectors Disk model: Samsung SSD 980 PRO 1TB Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disklabel type: gpt Disk identifier: ***

So now the question which ashift value should I use?
Can I maybe change my SSDs somehow to 4k or can it be that these SSDs only support 512?

I found no information about the sector size of the NVME Drives.... But I can't imagine that they only support 512.

Thanks in advance :)
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ashift=12 is still fine for 512B sector size, the only tradeoff being that you might waste some space, but this is negligible. I would also not necessarily trust the disk reporting 512B, often this is only done for compatibility purposes and internally the disks use 4K or even 8K sectors.
Generally you should go for ashift=12 - even on modern HDDs - unless you have good reasons to do otherwise.
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You can't. No manufacturer will tell you that in their datasheets. What you could do is installing PVE multiple times each time using another ashift and then run the same benchmark and compare the results. Using a ashift that is smaller than the internal block size should show worse performance in benchmarks.
So you for example could do 16K sequential sync writes/reads to a ashift of 9/12/13/14 ZFS pool and choose the ashift with the best performance.
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