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ssd slower read speed on / than on /home
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haVok
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PostPosted: Sun Dec 29, 2013 12:29 am    Post subject: ssd slower read speed on / than on /home Reply with quote

This seems strange to me, I have a /boot and luks partiton with lvm and root lv and home lv, the root lv with hdparm is showing an adverage of 345 mb read speed while the home lv is showing 480, I'm I missing something?
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frostschutz
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PostPosted: Sun Dec 29, 2013 1:15 am    Post subject: Reply with quote

Unusual for an SSD, if there is nothing special about the LV (such as active snapshots or whatever). But on the other hand, hdparm is not really a benchmark.

Device usage could make a difference, it may be able to return free space (zeroes) faster than actually reading any data.

Also, it may be something to do with caching.
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Hu
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PostPosted: Sun Dec 29, 2013 1:33 am    Post subject: Reply with quote

What block size are you using for the reads? Are all the partitions properly aligned?
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haVok
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PostPosted: Sun Dec 29, 2013 1:58 am    Post subject: Reply with quote

Well I'm not so keen on the hard drive stuff, but from what I understood fdisk should have aligned it properly
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Hu
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PostPosted: Sun Dec 29, 2013 3:02 am    Post subject: Reply with quote

If you mean fdisk from sys-apps/util-linux, the partitions are probably aligned for correct performance under DOS, meaning it is not aligned properly for SSDs.
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haVok
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PostPosted: Sun Dec 29, 2013 5:44 am    Post subject: Reply with quote

Well I decided to start over still not sure if they were align right. Booted parted magic to test hdparm and I get almost 500 mps on all partitons, hdparm on gentoo was showing way less.
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frostschutz
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PostPosted: Sun Dec 29, 2013 11:22 am    Post subject: Reply with quote

Hu wrote:
What block size are you using for the reads? Are all the partitions properly aligned?


If they're both LV in the same VG and same PV, their alignment can't be different from one another, as LVM itself is locked to its PE which is usually 4MB or larger.

So any alignment problem should affect both LVs the same. Not that alignment matters much in sequential reads as hdparm presumably does.

Of course it's still worth checking ;)
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Hu
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PostPosted: Sun Dec 29, 2013 4:37 pm    Post subject: Reply with quote

frostschutz wrote:
If they're both LV in the same VG and same PV, their alignment can't be different from one another, as LVM itself is locked to its PE which is usually 4MB or larger.
Good point. I missed that both LVs were on the same PV.
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