PSA PSA PSA: if you use LVM with USB drives you will get absolutely *abysmal* performance by default (especially on HDDs). You must use `pvcreate --dataalignment 2048s` to fix it. Fix requires re-creating PV and copying all data.
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Seriously, on most current drives (512e) random write performance will be *abso-fucking-lutely atrocious* unless you fix this, and SSDs will have increased wear.
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This happens because the USB drive maximum transfer size is 65535 sectors, and LVM in its infinite wisdom decides that's a good alignment value for PVs, thus de-aligning the physical sectors/FS blocks completely, causing read-modify-write cycles for every single small write.
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We're talking 20x performance hit kind of atrocious. 10 minutes for an rsync vs 30 seconds. Seriously. This is fucking bad.
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It seems this may only affect some enclosures (many/most UAS ones?). Check with `pvs -o +pe_start --units s`. If you get an odd or non-multiple-of-8 "1st PE" column then you're affected.
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Apparently querying /sys/block/$disk/queue/physical_block_size was too hard??
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The problem is they queried /sys/block/$disk/queue/optimal_io_size and trusted it over physical_block_size. And: # sg_inq -p 0xb0 /dev/sda [...] Optimal transfer length granularity: 8 blocks Maximum transfer length: 65535 blocks Optimal transfer length: 65535 blocks
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