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Filesystems - eMMC wearout mitigation #2774

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Filesystems - eMMC wearout mitigation #2774

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Filesystems - eMMC wearout mitigation
MichelMoriniaux Apr 11, 2019
46b6f96
Filesystems - eMMC wearout mitigation
MichelMoriniaux Apr 11, 2019
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24 changes: 21 additions & 3 deletions files/initramfs-tools/union-mount.j2
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Original file line number Diff line number Diff line change
Expand Up @@ -11,8 +11,22 @@ case $1 in
;;
esac

# Extract kernel parameters
set -- $(cat /proc/cmdline)
for x in "$@"; do
case "$x" in
loop=*)
image_dir=`echo $x | sed -e 's/.*loop=\(\S*\)\/.*/\1/'`
;;
varlog_size=*)
varlog_size="${x#varlog_size=}"
;;
*mmc_host*)
eMMC=1
esac
done

## Mount the overlay file system: rw layer over squashfs
image_dir=$(cat /proc/cmdline | sed -e 's/.*loop=\(\S*\)\/.*/\1/')
mkdir -p ${rootmnt}/host/$image_dir/rw
mkdir -p ${rootmnt}/host/$image_dir/work
mount -n -o lowerdir=${rootmnt},upperdir=${rootmnt}/host/$image_dir/rw,workdir=${rootmnt}/host/$image_dir/work -t overlay root-overlay ${rootmnt}
Expand All @@ -26,5 +40,9 @@ mount --bind ${rootmnt}/host/$image_dir/{{ DOCKERFS_DIR }} ${rootmnt}/var/lib/do
## Mount the boot directory in the raw partition, bypass the overlay
mkdir -p ${rootmnt}/boot
mount --bind ${rootmnt}/host/$image_dir/boot ${rootmnt}/boot
## Mount loop device for /var/log
[ -f ${rootmnt}/host/disk-img/var-log.ext4 ] && mount -t ext4 -o loop,rw ${rootmnt}/host/disk-img/var-log.ext4 ${rootmnt}/var/log
## Mount loop device for /var/log or a tmpfs device if the flash is eMMC
if [ -z ${eMMC+1} ]; then
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@jleveque jleveque Apr 11, 2019

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What is the purpose of the "+1" here?

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@MichelMoriniaux MichelMoriniaux Apr 11, 2019

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I originaly left it out but got pointed to https://stackoverflow.com/questions/3601515/how-to-check-if-a-variable-is-set-in-bash where it is stated that its a safer check

[ -f ${rootmnt}/host/disk-img/var-log.ext4 ] && mount -t ext4 -o loop,rw ${rootmnt}/host/disk-img/var-log.ext4 ${rootmnt}/var/log
else
mount -t tmpfs -o size=${varlog_size}M,mode=0755 tmpfs ${rootmnt}/var/log
fi
9 changes: 7 additions & 2 deletions files/initramfs-tools/varlog
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Expand Up @@ -17,6 +17,9 @@ for x in "$@"; do
case "$x" in
varlog_size=*)
varlog_size="${x#varlog_size=}"
;;
*mmc_host*)
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@jleveque jleveque Apr 11, 2019

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So this change depends on a "mmc_host" kernel command line parameter. I agree with your description that this is a crude check. Is there a way for a vendor to override this for their platform (or for an end-user building an image, for that matter) in the case it is not desired? What is populating the "mmc_host" kernel command line parameter?

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@MichelMoriniaux MichelMoriniaux Apr 11, 2019

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On my device the actual kernel command line parameter is:
block_flash=pci0000:00/0000:00:14.7/mmc_host/.*$
instead of looking for block_flash I'm looking for an actual MMC discriminator hence the *mmc_host*
again this may need more research to find a robust way of detecting that the image depends on flash and its not just a usb key with a eMMC component that is inserted and not used
Of all the platforms I have in my lab ( 6+ ) this is the only one I have found this.

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@MichelMoriniaux MichelMoriniaux Apr 11, 2019

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as for your question on overriding: maybe introduce a rules/config variable and check on that?
our vendor's recommendation for this change makes it clear that there is a real danger for the longevity of the switch if the default behavior is left as-is

eMMC=1
esac
done

Expand All @@ -32,5 +35,7 @@ if [ -e "${rootmnt}/host/disk-img/var-log.ext4" ]; then
fi
fi

# create varlog disk
mkdir -p ${rootmnt}/host/disk-img && ${rootmnt}/usr/bin/fallocate -l "$varlog_size"M ${rootmnt}/host/disk-img/var-log.ext4 && mkfs.ext4 -q -F ${rootmnt}/host/disk-img/var-log.ext4
# create varlog disk only if we are not running eMMC flash
if [ -z ${eMMC+1} ]; then
mkdir -p ${rootmnt}/host/disk-img && ${rootmnt}/usr/bin/fallocate -l "$varlog_size"M ${rootmnt}/host/disk-img/var-log.ext4 && mkfs.ext4 -q -F ${rootmnt}/host/disk-img/var-log.ext4
fi