"The ext4 code can read and write ext3 filesystems without any compatibility issues (i.e. none of the ext4-specific features are enabled automatically) and have been doing so for all RHEL and SLES users for the past several years via the CONFIG_EXT4_USE_FOR_EXT23 option (added in commit 24b58424 in 2009). It looks like the same for Ubuntu since early 2014 also.
The ext4-specific features either need to be enabled specifically via tune2fs (if possible) or at mkfs time."
és
"Both Red Hat and SuSE, as well as Debian and Ubuntu, are using ext4 with CONFIG_EXT4_USE_FOR_EXT23 for a couple of years now to support ext2 and ext3 file systems."
Vagyis már rég az ext4-et használjuk az ext3-hoz jó ideje.
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trey @ gépház