[OpenWrt-Devel] [PATCH 2/5] build: image: Add pad-to and pad-rootfs-squashfs helpers

Petr Štetiar ynezz at true.cz
Mon Apr 1 07:25:26 EDT 2019

Jo-Philipp Wich <jo at mein.io> [2019-04-01 11:18:55]:

> > Felix had a good point about sysupgrade, where we would needlesly write few
> > dozen MBs of 0s. In order to avoid that, I would simply add following qemu
> > variants to x86:
> I wonder what kind of storage media on x86 is so brittle that writing a
> few dozen MB of 0s will cause any noticeable effect.

SD cards could be slow. So it's going to make a difference if you're going to
write 20M or 273M combined image. On my apu2 with some noname SDHC SD card
(~6MB/s write max) I get ~15s difference in writting squashfs image
(unpadded=0.29s Vs padded=15s). 

I'm not sure if this is negligible or acceptable tradeoff, considering how
minor is the QEMU use case.

> Not opposed to that, but that nullifies the entire "save space on servers"
> argument. 

It wasn't my argument.

> Instead of having one set of images a few MB larger, we have a completely
> new set of larger images *and* the existing ones.

I'm all in for having usable images out of the box, without any additional
post-processing steps, but on the other hand I understand, that this might
slow down noticeably some existing use cases.

BTW, we're talking on x86 probably about additional 40M (~10M per each of 4
subtargets) and 8 additional QEMU ready images.  On armvirt/malta we're going
to produce(if agreed upon) padded images by default, so no difference there.

-- ynezz

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