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Re: [Qemu-block] [PATCH v5 2/5] qcow2: Document some maximum size constr
From: |
Alberto Garcia |
Subject: |
Re: [Qemu-block] [PATCH v5 2/5] qcow2: Document some maximum size constraints |
Date: |
Tue, 24 Apr 2018 11:13:47 +0200 |
User-agent: |
Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu) |
On Tue 24 Apr 2018 12:33:34 AM CEST, Eric Blake wrote:
> Although off_t permits up to 63 bits (8EB) of file offsets, in
> practice, we're going to hit other limits first. Document some
> of those limits in the qcow2 spec, and how choice of cluster size
> can influence some of the limits.
>
> While at it, notice that since we cannot map any virtual cluster
> to any address higher than 64 PB (56 bits) (due to the current L1/L2
> field encoding stopping at bit 55), it makes little sense to require
> the refcount table to access host offsets beyond that point. Mark
> the upper bits of the refcount table entries as reserved to match
> the L1/L2 table, with no ill effects, since it is unlikely that there
> are any existing images larger than 64PB in the first place, and thus
> all existing images already have those bits as 0. If 64PB proves to
> be too small in the future, we could enlarge all three uses of bit
> 55 into the reserved bits at that time. (For reference, ext4 with
> 4k blocks caps files at 16PB.)
>
> However, there is one limit that reserved bits don't help with: for
> compressed clusters, the L2 layout requires an ever-smaller maximum
> host offset as cluster size gets larger, down to a 512 TB maximum
> with 2M clusters.
>
> Signed-off-by: Eric Blake <address@hidden>
Reviewed-by: Alberto Garcia <address@hidden>
Berto
- [Qemu-block] [PATCH v5 0/5] minor qcow2 compression improvements, Eric Blake, 2018/04/23
- [Qemu-block] [PATCH v5 1/5] qcow2: Prefer byte-based calls into bs->file, Eric Blake, 2018/04/23
- [Qemu-block] [PATCH v5 4/5] qcow2: Don't allow overflow during cluster allocation, Eric Blake, 2018/04/23
- [Qemu-block] [PATCH v5 2/5] qcow2: Document some maximum size constraints, Eric Blake, 2018/04/23
- Re: [Qemu-block] [PATCH v5 2/5] qcow2: Document some maximum size constraints,
Alberto Garcia <=
- [Qemu-block] [PATCH v5 3/5] qcow2: Reduce REFT_OFFSET_MASK, Eric Blake, 2018/04/23
- [Qemu-block] [PATCH v5 5/5] qcow2: Avoid memory over-allocation on compressed images, Eric Blake, 2018/04/23