breaking(general): remove support for LZ4 (#5296)

- It has been deprecated in Kopia since July 2022 (0985b80). - Dependency appears unmaintained.
2026-05-09 15:23:02 -04:00 · 2026-04-16 15:13:17 -07:00
parent c9334c9bc5
commit be2ce68b5c
8 changed files with 61 additions and 105 deletions
--- a/go.mod
+++ b/go.mod
@@ -38,7 +38,6 @@ require (
 	github.com/mxk/go-vss v1.2.1
 	github.com/natefinch/atomic v1.0.1
 	github.com/petar/GoLLRB v0.0.0-20210522233825-ae3b015fd3e9
-	github.com/pierrec/lz4 v2.6.1+incompatible
 	github.com/pkg/errors v0.9.1
 	github.com/pkg/profile v1.7.0
 	github.com/pkg/sftp v1.13.10
@@ -99,7 +98,6 @@ require (
 	github.com/envoyproxy/protoc-gen-validate v1.3.0 // indirect
 	github.com/felixge/fgprof v0.9.3 // indirect
 	github.com/felixge/httpsnoop v1.0.4 // indirect
-	github.com/frankban/quicktest v1.13.1 // indirect
 	github.com/go-ini/ini v1.67.0 // indirect
 	github.com/go-jose/go-jose/v4 v4.1.4 // indirect
 	github.com/go-json-experiment/json v0.0.0-20250725192818-e39067aee2d2 // indirect
--- a/go.sum
+++ b/go.sum
@@ -72,7 +72,6 @@ github.com/cncf/xds/go v0.0.0-20251210132809-ee656c7534f5 h1:6xNmx7iTtyBRev0+D/T
 github.com/cncf/xds/go v0.0.0-20251210132809-ee656c7534f5/go.mod h1:KdCmV+x/BuvyMxRnYBlmVaq4OLiKW6iRQfvC62cvdkI=
 github.com/coreos/go-systemd/v22 v22.7.0 h1:LAEzFkke61DFROc7zNLX/WA2i5J8gYqe0rSj9KI28KA=
 github.com/coreos/go-systemd/v22 v22.7.0/go.mod h1:xNUYtjHu2EDXbsxz1i41wouACIwT7Ybq9o0BQhMwD0w=
-github.com/creack/pty v1.1.9/go.mod h1:oKZEueFk5CKHvIhNR5MUki03XCEU+Q6VDXinZuGJ33E=
 github.com/danieljoos/wincred v1.2.3 h1:v7dZC2x32Ut3nEfRH+vhoZGvN72+dQ/snVXo/vMFLdQ=
 github.com/danieljoos/wincred v1.2.3/go.mod h1:6qqX0WNrS4RzPZ1tnroDzq9kY3fu1KwE7MRLQK4X0bs=
 github.com/davecgh/go-spew v0.0.0-20161028175848-04cdfd42973b/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
@@ -102,8 +101,6 @@ github.com/felixge/httpsnoop v1.0.4 h1:NFTV2Zj1bL4mc9sqWACXbQFVBBg2W3GPvqp8/ESS2
 github.com/felixge/httpsnoop v1.0.4/go.mod h1:m8KPJKqk1gH5J9DgRY2ASl2lWCfGKXixSwevea8zH2U=
 github.com/foomo/htpasswd v0.0.0-20200116085101-e3a90e78da9c h1:DBGU7zCwrrPPDsD6+gqKG8UfMxenWg9BOJE/Nmfph+4=
 github.com/foomo/htpasswd v0.0.0-20200116085101-e3a90e78da9c/go.mod h1:SHawtolbB0ZOFoRWgDwakX5WpwuIWAK88bUXVZqK0Ss=
-github.com/frankban/quicktest v1.13.1 h1:xVm/f9seEhZFL9+n5kv5XLrGwy6elc4V9v/XFY2vmd8=
-github.com/frankban/quicktest v1.13.1/go.mod h1:NeW+ay9A/U67EYXNFA1nPE8e/tnQv/09mUdL/ijj8og=
 github.com/go-ini/ini v1.67.0 h1:z6ZrTEZqSWOTyH2FlglNbNgARyHG8oLW9gMELqKr06A=
 github.com/go-ini/ini v1.67.0/go.mod h1:ByCAeIL28uOIIG0E3PJtZPDL8WnHpFKFOtgjp+3Ies8=
 github.com/go-jose/go-jose/v4 v4.1.4 h1:moDMcTHmvE6Groj34emNPLs/qtYXRVcd6S7NHbHz3kA=
@@ -137,7 +134,6 @@ github.com/golang/protobuf v1.5.4 h1:i7eJL8qZTpSEXOPTxNKhASYpMn+8e5Q6AdndVa1dWek
 github.com/golang/protobuf v1.5.4/go.mod h1:lnTiLA8Wa4RWRcIUkrtSVa5nRhsEGBg48fD6rSs7xps=
 github.com/google/fswalker v0.3.3 h1:K2+d6cb3vNFjquVPRObIY+QaXJ6cbleVV6yZWLzkkQ8=
 github.com/google/fswalker v0.3.3/go.mod h1:9upMSscEE8oRi0WJ0rXZZYya1DmgUtJFhXAw7KNS3c4=
-github.com/google/go-cmp v0.5.6/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
 github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
 github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
 github.com/google/martian/v3 v3.3.3 h1:DIhPTQrbPkgs2yJYdXU/eNACCG5DVQjySNRNlflZ9Fc=
@@ -181,12 +177,8 @@ github.com/kopia/htmluibuild v0.0.1-0.20260414002305-a859d43ee893 h1:J9g3qLDJAIe
 github.com/kopia/htmluibuild v0.0.1-0.20260414002305-a859d43ee893/go.mod h1:h53A5JM3t2qiwxqxusBe+PFgGcgZdS+DWCQvG5PTlto=
 github.com/kr/fs v0.1.0 h1:Jskdu9ieNAYnjxsi0LbQp1ulIKZV1LAFgK1tWhpZgl8=
 github.com/kr/fs v0.1.0/go.mod h1:FFnZGqtBN9Gxj7eW1uZ42v5BccTP0vu6NEaFoC2HwRg=
-github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORNo=
-github.com/kr/pretty v0.3.0/go.mod h1:640gp4NfQd8pI5XOwp5fnNeVWj67G7CFk/SaSQn7NBk=
 github.com/kr/pretty v0.3.1 h1:flRD4NNwYAUpkphVc1HcthR4KEIFJ65n8Mw5qdRn3LE=
 github.com/kr/pretty v0.3.1/go.mod h1:hoEshYVHaxMs3cyo3Yncou5ZscifuDolrwPKZanG3xk=
-github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
-github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
 github.com/kr/text v0.2.0 h1:5Nx0Ya0ZqY2ygV366QzturHI13Jq95ApcVaJBhpS+AY=
 github.com/kr/text v0.2.0/go.mod h1:eLer722TekiGuMkidMxC/pM04lWEeraHUUmBw8l2grE=
 github.com/kylelemons/godebug v1.1.0 h1:RPNrshWIDI6G2gRW9EHilWtl7Z6Sb1BR0xunSBf0SNc=
@@ -219,8 +211,6 @@ github.com/petar/GoLLRB v0.0.0-20210522233825-ae3b015fd3e9 h1:1/WtZae0yGtPq+TI6+
 github.com/petar/GoLLRB v0.0.0-20210522233825-ae3b015fd3e9/go.mod h1:x3N5drFsm2uilKKuuYo6LdyD8vZAW55sH/9w+pbo1sw=
 github.com/philhofer/fwd v1.2.0 h1:e6DnBTl7vGY+Gz322/ASL4Gyp1FspeMvx1RNDoToZuM=
 github.com/philhofer/fwd v1.2.0/go.mod h1:RqIHx9QI14HlwKwm98g9Re5prTQ6LdeRQn+gXJFxsJM=
-github.com/pierrec/lz4 v2.6.1+incompatible h1:9UY3+iC23yxF0UfGaYrGplQ+79Rg+h/q9FV9ix19jjM=
-github.com/pierrec/lz4 v2.6.1+incompatible/go.mod h1:pdkljMzZIN41W+lC3N2tnIh5sFi+IEE17M5jbnwPHcY=
 github.com/pkg/browser v0.0.0-20240102092130-5ac0b6a4141c h1:+mdjkGKdHQG3305AYmdv1U2eRNDiU2ErMBj1gwrq8eQ=
 github.com/pkg/browser v0.0.0-20240102092130-5ac0b6a4141c/go.mod h1:7rwL4CYBLnjLxUqIJNnCWiEdr3bn6IUYi15bNlnbCCU=
 github.com/pkg/errors v0.9.1 h1:FEBLx1zS214owpjy7qsBeixbURkuhQAwrK5UwLGTwt4=
@@ -245,7 +235,6 @@ github.com/prometheus/common v0.67.5 h1:pIgK94WWlQt1WLwAC5j2ynLaBRDiinoAb86HZHTU
 github.com/prometheus/common v0.67.5/go.mod h1:SjE/0MzDEEAyrdr5Gqc6G+sXI67maCxzaT3A2+HqjUw=
 github.com/prometheus/procfs v0.16.1 h1:hZ15bTNuirocR6u0JZ6BAHHmwS1p8B4P6MRqxtzMyRg=
 github.com/prometheus/procfs v0.16.1/go.mod h1:teAbpZRB1iIAJYREa1LsoWUXykVXA1KlTmWl8x/U+Is=
-github.com/rogpeppe/go-internal v1.6.1/go.mod h1:xXDCJY+GAPziupqXw64V24skbSoqbTEfhy4qGm1nDQc=
 github.com/rogpeppe/go-internal v1.14.1 h1:UQB4HGPB6osV0SQTLymcB4TgvyWu6ZyliaW0tI/otEQ=
 github.com/rogpeppe/go-internal v1.14.1/go.mod h1:MaRKkUm5W0goXpeCfT7UZI6fk/L7L7so1lCWt35ZSgc=
 github.com/rs/xid v1.6.0 h1:fV591PaemRlL6JfRxGDEPl69wICngIQ3shQtzfy2gxU=
@@ -349,7 +338,6 @@ golang.org/x/text v0.36.0 h1:JfKh3XmcRPqZPKevfXVpI1wXPTqbkE5f7JA92a55Yxg=
 golang.org/x/text v0.36.0/go.mod h1:NIdBknypM8iqVmPiuco0Dh6P5Jcdk8lJL0CUebqK164=
 golang.org/x/time v0.15.0 h1:bbrp8t3bGUeFOx08pvsMYRTCVSMk89u4tKbNOZbp88U=
 golang.org/x/time v0.15.0/go.mod h1:Y4YMaQmXwGQZoFaVFk4YpCt4FLQMYKZe9oeV/f4MSno=
-golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 gonum.org/v1/gonum v0.17.0 h1:VbpOemQlsSMrYmn7T2OUvQ4dqxQXU+ouZFQsZOx50z4=
 gonum.org/v1/gonum v0.17.0/go.mod h1:El3tOrEuMpv2UdMrbNlKEh9vd86bmQ6vqIcDwxEOc1E=
 google.golang.org/api v0.275.0 h1:vfY5d9vFVJeWEZT65QDd9hbndr7FyZ2+6mIzGAh71NI=
@@ -365,10 +353,8 @@ google.golang.org/grpc v1.80.0/go.mod h1:ho/dLnxwi3EDJA4Zghp7k2Ec1+c2jqup0bFkw07
 google.golang.org/protobuf v1.36.11 h1:fV6ZwhNocDyBLK0dj+fg8ektcVegBBuEolpbTQyBNVE=
 google.golang.org/protobuf v1.36.11/go.mod h1:HTf+CrKn2C3g5S8VImy6tdcUvCska2kB7j23XfzDpco=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
-gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c h1:Hei/4ADfdWqJk1ZMxUNpqntNwaWcugrBjAiHlqqRiVk=
 gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c/go.mod h1:JHkPIbrfpd72SG/EVd6muEfDQjcINNoR0C8j2r3qZ4Q=
-gopkg.in/errgo.v2 v2.1.0/go.mod h1:hNsd1EY+bozCKY1Ytp96fpM3vjJbqLJn88ws8XvfDNI=
 gopkg.in/kothar/go-backblaze.v0 v0.0.0-20210124194846-35409b867216 h1:2TSTkQ8PMvGOD5eeqqRVv6Z9+BYI+bowK97RCr3W+9M=
 gopkg.in/kothar/go-backblaze.v0 v0.0.0-20210124194846-35409b867216/go.mod h1:zJ2QpyDCYo1KvLXlmdnFlQAyF/Qfth0fB8239Qg7BIE=
 gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
--- a/repo/compression/compression_ids.go
+++ b/repo/compression/compression_ids.go
@@ -23,7 +23,7 @@
 	headerPgzipBestSpeed       HeaderID = 0x1301
 	headerPgzipBestCompression HeaderID = 0x1302

-	headerLZ4Default HeaderID = 0x1400
+	headerLZ4Removed HeaderID = 0x1400 // historically used for LZ4 and must not be reused.

 	headerDeflateDefault         HeaderID = 0x1500
 	headerDeflateBestSpeed       HeaderID = 0x1501
--- a/repo/compression/compressor.go
+++ b/repo/compression/compressor.go
@@ -32,6 +32,7 @@ type Compressor interface {
 	ByName         = map[Name]Compressor{}
 	HeaderIDToName = map[HeaderID]Name{}
 	IsDeprecated   = map[Name]bool{}
+	isUnsupported  = map[HeaderID]bool{}
 )

 // RegisterCompressor registers the provided compressor implementation.
@@ -56,6 +57,13 @@ func RegisterDeprecatedCompressor(name Name, c Compressor) {
 	IsDeprecated[name] = true
 }

+func registerUnsupportedCompressor(name Name, c Compressor) {
+	RegisterCompressor(name, c)
+
+	IsDeprecated[name] = true
+	isUnsupported[c.HeaderID()] = true
+}
+
 func compressionHeader(id HeaderID) []byte {
 	b := make([]byte, compressionHeaderSize)
 	binary.BigEndian.PutUint32(b, uint32(id))
@@ -81,6 +89,17 @@ func DecompressByHeader(output io.Writer, input io.Reader) error {
 	return errors.Wrap(compressor.Decompress(output, input, false), "error decompressing")
 }

+// IsSupported returns whether a named compression scheme is supported.
+func IsSupported(name Name) bool {
+	c := ByName[name]
+
+	if c == nil {
+		return false
+	}
+
+	return !isUnsupported[c.HeaderID()]
+}
+
 func mustSucceed(err error) {
 	impossible.PanicOnError(err)
 }
--- a/repo/compression/compressor_lz4.go
+++ b/repo/compression/compressor_lz4.go
@@ -1,82 +1,26 @@
 package compression

 import (
+	"errors"
 	"io"
-	"sync"
-
-	"github.com/pierrec/lz4"
-	"github.com/pkg/errors"
-
-	"github.com/kopia/kopia/internal/freepool"
-	"github.com/kopia/kopia/internal/iocopy"
 )

 func init() {
-	RegisterDeprecatedCompressor("lz4", newLZ4Compressor(headerLZ4Default))
+	registerUnsupportedCompressor("lz4", lz4Compressor{})
 }

-func newLZ4Compressor(id HeaderID) Compressor {
-	return &lz4Compressor{id, compressionHeader(id), sync.Pool{
-		New: func() any {
-			return lz4.NewWriter(io.Discard)
-		},
-	}}
+var errLZ4NotSupported = errors.New("LZ4 compressor is not supported in recent versions of kopia, version v0.22.3 or older is needed to read legacy repositories that use the LZ4 compressor")
+
+type lz4Compressor struct{}
+
+func (c lz4Compressor) HeaderID() HeaderID {
+	return headerLZ4Removed
 }

-type lz4Compressor struct {
-	id     HeaderID
-	header []byte
-	pool   sync.Pool
+func (c lz4Compressor) Compress(_ io.Writer, _ io.Reader) error {
+	return errLZ4NotSupported
 }

-func (c *lz4Compressor) HeaderID() HeaderID {
-	return c.id
-}
-
-func (c *lz4Compressor) Compress(output io.Writer, input io.Reader) error {
-	if _, err := output.Write(c.header); err != nil {
-		return errors.Wrap(err, "unable to write header")
-	}
-
-	//nolint:forcetypeassert
-	w := c.pool.Get().(*lz4.Writer)
-	defer c.pool.Put(w)
-
-	w.Reset(output)
-
-	if err := iocopy.JustCopy(w, input); err != nil {
-		return errors.Wrap(err, "compression error")
-	}
-
-	if err := w.Close(); err != nil {
-		return errors.Wrap(err, "compression close error")
-	}
-
-	return nil
-}
-
-//nolint:gochecknoglobals
-var lz4DecoderPool = freepool.New(func() *lz4.Reader {
-	return lz4.NewReader(nil)
-}, func(v *lz4.Reader) {
-	v.Reset(nil)
-})
-
-func (c *lz4Compressor) Decompress(output io.Writer, input io.Reader, withHeader bool) error {
-	if withHeader {
-		if err := verifyCompressionHeader(input, c.header); err != nil {
-			return err
-		}
-	}
-
-	dec := lz4DecoderPool.Take()
-	defer lz4DecoderPool.Return(dec)
-
-	dec.Reset(input)
-
-	if err := iocopy.JustCopy(output, dec); err != nil {
-		return errors.Wrap(err, "decompression error")
-	}
-
-	return nil
+func (c lz4Compressor) Decompress(_ io.Writer, _ io.Reader, _ bool) error {
+	return errLZ4NotSupported
 }
--- a/repo/compression/compressor_test.go
+++ b/repo/compression/compressor_test.go
@@ -14,6 +14,10 @@ func TestMain(m *testing.M) { testutil.MyTestMain(m) }

 func TestCompressor(t *testing.T) {
 	for id, comp := range ByHeaderID {
+		if isUnsupported[id] {
+			continue
+		}
+
 		t.Run(fmt.Sprintf("compressible-data-%x", id), func(t *testing.T) {
 			// make sure all-zero data is compressed
 			data := make([]byte, 10000)
--- a/repo/object/object_manager_test.go
+++ b/repo/object/object_manager_test.go
@@ -802,6 +802,10 @@ func TestEndToEndReadAndSeekWithCompression(t *testing.T) {

 	for _, compressible := range []bool{false, true} {
 		for compressorName := range compression.ByName {
+			if !compression.IsSupported(compressorName) {
+				continue
+			}
+
 			t.Run(string(compressorName), func(t *testing.T) {
 				ctx := testlogging.Context(t)

--- a/site/content/docs/Advanced/Compression/_index.md
+++ b/site/content/docs/Advanced/Compression/_index.md
@@ -35,18 +35,17 @@ Repeating 1 times per compression method (total 466.7 MiB).
  3. s2-parallel-4              127.1 MiB    2.3 GiB/s    2951   344.1 MiB
  4. pgzip-best-speed           96.7 MiB     2.1 GiB/s    4127   324.1 MiB
  5. pgzip                      86.3 MiB     1.2 GiB/s    4132   298.7 MiB
-  6. lz4                        131.8 MiB    458.9 MiB/s  17     321.7 MiB
-  7. zstd-fastest               79.8 MiB     356.2 MiB/s  22503  246 MiB
-  8. zstd                       76.8 MiB     323.7 MiB/s  22605  237.8 MiB
-  9. deflate-best-speed         96.7 MiB     220.8 MiB/s  45     310.8 MiB
- 10. gzip-best-speed            94.9 MiB     165 MiB/s    40     305.2 MiB
- 11. deflate-default            86.3 MiB     143.1 MiB/s  34     311 MiB
- 12. zstd-better-compression    74.2 MiB     104 MiB/s    22496  251.4 MiB
- 13. pgzip-best-compression     83 MiB       55.9 MiB/s   4359   299.1 MiB
- 14. gzip                       83.6 MiB     40.5 MiB/s   69     304.8 MiB
- 15. zstd-best-compression      68.9 MiB     19.2 MiB/s   22669  303.4 MiB
- 16. deflate-best-compression   83 MiB       5.6 MiB/s    134    311 MiB
- 17. gzip-best-compression      83 MiB       5.1 MiB/s    137    304.8 MiB
+  6. zstd-fastest               79.8 MiB     356.2 MiB/s  22503  246 MiB
+  7. zstd                       76.8 MiB     323.7 MiB/s  22605  237.8 MiB
+  8. deflate-best-speed         96.7 MiB     220.8 MiB/s  45     310.8 MiB
+  9. gzip-best-speed            94.9 MiB     165 MiB/s    40     305.2 MiB
+ 10. deflate-default            86.3 MiB     143.1 MiB/s  34     311 MiB
+ 11. zstd-better-compression    74.2 MiB     104 MiB/s    22496  251.4 MiB
+ 12. pgzip-best-compression     83 MiB       55.9 MiB/s   4359   299.1 MiB
+ 13. gzip                       83.6 MiB     40.5 MiB/s   69     304.8 MiB
+ 14. zstd-best-compression      68.9 MiB     19.2 MiB/s   22669  303.4 MiB
+ 15. deflate-best-compression   83 MiB       5.6 MiB/s    134    311 MiB
+ 16. gzip-best-compression      83 MiB       5.1 MiB/s    137    304.8 MiB
 ```

 As you can see, s2 compression clearly favors performance over compression ratio. zstd on the other hand results almost half the size as s2. pgzip arguably offers the best balance on two worlds.
@@ -73,21 +72,23 @@ Repeating 100 times per compression method (total 12.5 MiB).
  8. gzip-best-speed            33.7 KiB     150.6 MiB/s  28     1.2 MiB
  9. pgzip-best-speed           34.3 KiB     143.7 MiB/s  1649   220.2 MiB
 10. deflate-default            31.2 KiB     126.3 MiB/s  22     1.1 MiB
- 11. lz4                        44.7 KiB     112.6 MiB/s  435    816.7 MiB
- 12. pgzip                      31.2 KiB     94.6 MiB/s   2634   277.5 MiB
- 13. gzip                       30.4 KiB     39.5 MiB/s   26     874.7 KiB
- 14. deflate-best-compression   30.4 KiB     25.4 MiB/s   21     1 MiB
- 15. gzip-best-compression      30.4 KiB     24.5 MiB/s   27     874.9 KiB
- 16. pgzip-best-compression     30.4 KiB     23.1 MiB/s   2646   281.8 MiB
- 17. zstd-best-compression      25.1 KiB     19.3 MiB/s   882    99.3 MiB
+ 11. pgzip                      31.2 KiB     94.6 MiB/s   2634   277.5 MiB
+ 12. gzip                       30.4 KiB     39.5 MiB/s   26     874.7 KiB
+ 13. deflate-best-compression   30.4 KiB     25.4 MiB/s   21     1 MiB
+ 14. gzip-best-compression      30.4 KiB     24.5 MiB/s   27     874.9 KiB
+ 15. pgzip-best-compression     30.4 KiB     23.1 MiB/s   2646   281.8 MiB
+ 16. zstd-best-compression      25.1 KiB     19.3 MiB/s   882    99.3 MiB
 ```

-While s2 significantly uses way less memory in this case, pgzip's numbers seem indifferent to the input size. [Turns out](https://github.com/klauspost/pgzip/issues/44), pgzip has different memory usage logic. It would quickly allocate necessary memory and plateau, which s2 is more on a linear fashion. Here is rough graph is demonstrate the difference:
+While s2 uses significantly less memory in this case, pgzip's numbers seem indifferent to the input size. [Turns out](https://github.com/klauspost/pgzip/issues/44), pgzip has different memory usage logic. It would quickly allocate necessary memory and plateau, which s2 is more on a linear fashion. Here is rough graph is demonstrate the difference:

 ![s2 vs pgzip](s2_vs_pgzip.svg)

 Therefore, if your backup target is small, and memory is extremely restricted, s2 might be necessary. Otherwise, all algorithms are valid candidates.

+ Note: Newer Kopia versions no longer support reading contents that were compressed with the deprecated LZ4 algorithm. If your repository contains data written with LZ4, you must migrate it first using a Kopia version that still supports LZ4—for example by restoring the affected snapshots and/or repacking the repository with one of the currently supported compression algorithms—before upgrading.
+
+
 ### Minimum file size and extensions to compress

 As discussed above, some compression algorithms make sense only if the payload is large enough. So it might be beneficial to set a minimum file size when using these algorithms.