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Subject: Re: Hot data tracking / hybrid storage
To: "Austin S. Hemmelgarn" <ahferroin7@gmail.com>,
        Ferry Toth <ftoth@exalondelft.nl>, linux-btrfs@vger.kernel.org
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From: Andrei Borzenkov <arvidjaar@gmail.com>
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Date: Sun, 29 May 2016 09:23:59 +0300
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20.05.2016 20:59, Austin S. Hemmelgarn пишет:
> On 2016-05-20 13:02, Ferry Toth wrote:
>> We have 4 1TB drives in MBR, 1MB free at the beginning, grub on all 4,
>> then 8GB swap, then all the rest btrfs (no LVM used). The 4 btrfs
>> partitions are in the same pool, which is in btrfs RAID10 format. /boot
>> is in subvolume @boot.
> If you have GRUB installed on all 4, then you don't actually have the
> full 2047 sectors between the MBR and the partition free, as GRUB is
> embedded in that space.  I forget exactly how much space it takes up,
> but I know it's not the whole 1023.5K  I would not suggest risking usage
> of the final 8k there though.

If you mean grub2, required space is variable and depends on where
/boot/grub is located (i.e. which drivers it needs to access it).
Assuming plain btrfs on legacy BIOS MBR, required space is around 40-50KB.

Note that grub2 detects some post-MBR gap software signatures and skips
over them (space need not be contiguous). It is entirely possible to add
bcache detection if enough demand exists.