From: Zdenek Kabelac <firstname.lastname@example.org> To: "LVM general discussion and development" <email@example.com>, "Tomas Dalebjörk" <firstname.lastname@example.org> Subject: Re: [linux-lvm] how to convert a disk containing a snapshot to a snapshot lv? Date: Tue, 21 Dec 2021 14:46:37 +0100 [thread overview] Message-ID: <email@example.com> (raw) In-Reply-To: <CACrcyf+2uz_ngeVdq48WjSg6ZaarRpVXs8TZwzpQ4hnW7D7xGg@mail.gmail.com> Dne 19. 12. 21 v 17:43 Tomas Dalebjörk napsal(a): > Hi, > > I am trying to understand how to convert a disk containing snapshot data. > This is how I tested this: > 1. locate the snapshot testlv.211218.232255 > root@debian10:/dev/mapper# lvs > LV VG Attr LSize Pool Origin Data% > Meta% Move Log Cpy%Sync Convert > home debian10-vg -wi-ao---- 1.00g > root debian10-vg -wi-ao---- <3.81g > swap_1 debian10-vg -wi-ao---- 976.00m > testlv debian10-vg owi-aos--- 100.00m > testlv.211218.232255 debian10-vg swi-a-s--- 104.00m testlv 1.44 > root@debian10:/dev/mapper# > Hi It looks like there is some sort of misunderstanding HOW these snapshots with lvm2 do work. The snapshot LV does NOT contain whole copy of the origin LV (the one you've take its snapshot) So while 'lvs' is presenting you the size of snapshot - it's not a 'regular' volume behind the scene. Instead it's a set of differentiating blocks from your origin stored in a way the came in use. So while your snapshot looks it has size 104Mib and origin 100MiB - it means there is just allocated max possible size of snapshot to store all different chunk + some extra metadata. But ATM only 1.44% of this space is actually used to hold all these block (AKA ~1.5MiB or real storage was used so far) There is basically NO WAY to use this 'COW' storage area without its original volume. If you want to use such 'snapshot' elsewhere you simply need to *dd* such LV - which is presented (via magic world of DM targets) to user-space to appear like your original LV in the moment of snapshot. But the actual 'raw' content stored in disk is in 'internal' form - not really usable outside (although the format is not really a complicated one) Note - these so called 'thick' snapshot or 'old' snapshot are now better handled via thin-provisioning - giving much better performance - especially if you plan to keep snapshot for long term or have it in bigger size. Hopefully it now makes it way more clear. Regards Zdenek _______________________________________________ linux-lvm mailing list firstname.lastname@example.org https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
next prev parent reply other threads:[~2021-12-21 13:47 UTC|newest] Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-12-19 16:43 Tomas Dalebjörk 2021-12-20 14:51 ` Phillip Susi 2021-12-21 13:46 ` Zdenek Kabelac [this message] 2021-12-21 14:44 ` Tomas Dalebjörk 2021-12-21 15:30 ` Zdenek Kabelac 2021-12-21 16:12 ` Tomas Dalebjörk 2021-12-22 10:40 ` Zdenek Kabelac 2021-12-28 5:04 ` Stuart D Gathman 2021-12-28 6:40 ` Tomas Dalebjörk 2021-12-29 13:35 ` Stuart D Gathman 2021-12-21 19:23 ` Phillip Susi
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