diff --git a/ais/tgtfcold.go b/ais/tgtfcold.go
index 516707bc520..e2186f8950f 100644
--- a/ais/tgtfcold.go
+++ b/ais/tgtfcold.go
@@ -6,7 +6,6 @@ package ais
 
 import (
 	"io"
-	"os"
 
 	"github.com/NVIDIA/aistore/ais/s3"
 	"github.com/NVIDIA/aistore/cmn"
@@ -28,7 +27,7 @@ func (goi *getOI) coldReopen(res *core.GetReaderResult) error {
 	var (
 		err    error
 		lmfh   cos.LomReader
-		wfh    *os.File
+		wfh    cos.LomWriter
 		t      = goi.t
 		lom    = goi.lom
 		revert string
diff --git a/ais/tgtobj.go b/ais/tgtobj.go
index 239be3f8e6a..54946de4d02 100644
--- a/ais/tgtobj.go
+++ b/ais/tgtobj.go
@@ -1251,14 +1251,14 @@ func (a *apndOI) apnd(buf []byte) (packedHdl string, ecode int, err error) {
 				ecode = http.StatusInternalServerError
 				return
 			}
-			fh, err = os.OpenFile(workFQN, os.O_APPEND|os.O_WRONLY, cos.PermRWR)
+			fh, err = os.OpenFile(workFQN, os.O_APPEND|os.O_WRONLY, cos.PermRWR) // O_APPEND
 		} else {
 			a.lom.Unlock(false)
 			a.hdl.partialCksum = cos.NewCksumHash(a.lom.CksumType())
 			fh, err = a.lom.CreateWork(workFQN)
 		}
 	} else {
-		fh, err = os.OpenFile(workFQN, os.O_APPEND|os.O_WRONLY, cos.PermRWR)
+		fh, err = os.OpenFile(workFQN, os.O_APPEND|os.O_WRONLY, cos.PermRWR) // O_APPEND
 		debug.Assert(a.hdl.partialCksum != nil)
 	}
 	if err != nil { // failed to open or create
diff --git a/ais/tgts3mpt.go b/ais/tgts3mpt.go
index 6fbcc00d9c5..70ff39e4848 100644
--- a/ais/tgts3mpt.go
+++ b/ais/tgts3mpt.go
@@ -122,7 +122,7 @@ func (t *target) putMptPart(w http.ResponseWriter, r *http.Request, items []stri
 	// workfile name format: <upload-id>.<part-number>.<obj-name>
 	prefix := uploadID + "." + strconv.FormatInt(int64(partNum), 10)
 	wfqn := fs.CSM.Gen(lom, fs.WorkfileType, prefix)
-	partFh, errC := lom.CreateWorkRW(wfqn)
+	partFh, errC := lom.CreatePart(wfqn)
 	if errC != nil {
 		s3.WriteMptErr(w, r, errC, 0, lom, uploadID)
 		return
diff --git a/cmn/cos/io.go b/cmn/cos/io.go
index 9ddc84cca31..a59e4ad3ef3 100644
--- a/cmn/cos/io.go
+++ b/cmn/cos/io.go
@@ -33,32 +33,30 @@ const ContentLengthUnknown = -1
 
 const PathSeparator = string(filepath.Separator)
 
-// readers
+// follows below:
+// - readers: interfaces
+// - readers: implementations
+// - handles
+// - writers
+
+// readers: interfaces
 type (
 	ReadOpenCloser interface {
 		io.ReadCloser
 		Open() (ReadOpenCloser, error)
 	}
-	// ReadSizer is the interface that adds Size method to io.Reader.
 	ReadSizer interface {
 		io.Reader
 		Size() int64
 	}
-	// ReadCloseSizer is the interface that adds Size method to io.ReadCloser.
 	ReadCloseSizer interface {
 		io.ReadCloser
 		Size() int64
 	}
-	// ReadOpenCloseSizer is the interface that adds Size method to ReadOpenCloser.
-	ReadOpenCloseSizer interface {
+	ReadOpenCloseSizer interface { // see sizedReader below
 		ReadOpenCloser
 		Size() int64
 	}
-	sizedReader struct {
-		io.Reader
-		size int64
-	}
-
 	ReadReaderAt interface {
 		io.Reader
 		io.ReaderAt
@@ -67,8 +65,18 @@ type (
 		io.ReadCloser
 		io.ReaderAt
 	}
+	LomWriter interface {
+		io.WriteCloser
+		Sync() error
+	}
+)
 
-	// implementations
+// readers: implementations
+type (
+	sizedReader struct {
+		io.Reader
+		size int64
+	}
 
 	nopReader struct {
 		size   int
@@ -100,7 +108,7 @@ type (
 	nopOpener struct{ io.ReadCloser }
 )
 
-// handles (and more readers)
+// handles (and even more readers)
 type (
 	FileHandle struct {
 		*os.File
diff --git a/core/lfile.go b/core/lfile.go
index 59ec30e341f..ec1adc3d7c9 100644
--- a/core/lfile.go
+++ b/core/lfile.go
@@ -26,21 +26,19 @@ func (lom *LOM) Open() (cos.LomReader, error) {
 // create
 //
 
-func (lom *LOM) Create() (*os.File, error) {
+func (lom *LOM) Create() (cos.LomWriter, error) {
 	debug.Assert(lom.isLockedExcl(), lom.Cname()) // caller must wlock
-	return lom._cf(lom.FQN, os.O_WRONLY)
+	return lom._cf(lom.FQN)
 }
 
-func (lom *LOM) CreateWork(wfqn string) (*os.File, error) {
-	return lom._cf(wfqn, os.O_WRONLY)
-}
+func (lom *LOM) CreateWork(wfqn string) (*os.File, error)  { return lom._cf(wfqn) } // -> lom
+func (lom *LOM) CreatePart(wfqn string) (*os.File, error)  { return lom._cf(wfqn) } // TODO: differentiate
+func (lom *LOM) CreateSlice(wfqn string) (*os.File, error) { return lom._cf(wfqn) } // TODO: ditto
 
-func (lom *LOM) CreateWorkRW(wfqn string) (*os.File, error) {
-	return lom._cf(wfqn, os.O_RDWR)
-}
+const _openFlags = os.O_CREATE | os.O_WRONLY | os.O_TRUNC
 
-func (lom *LOM) _cf(fqn string, mode int) (fh *os.File, err error) {
-	fh, err = os.OpenFile(fqn, os.O_CREATE|mode|os.O_TRUNC, cos.PermRWR)
+func (lom *LOM) _cf(fqn string) (fh *os.File, err error) {
+	fh, err = os.OpenFile(fqn, _openFlags, cos.PermRWR)
 	if err == nil || !os.IsNotExist(err) {
 		return fh, err
 	}
@@ -54,7 +52,7 @@ func (lom *LOM) _cf(fqn string, mode int) (fh *os.File, err error) {
 	if err = cos.CreateDir(fdir); err != nil {
 		return nil, err
 	}
-	return os.OpenFile(fqn, os.O_CREATE|mode|os.O_TRUNC, cos.PermRWR)
+	return os.OpenFile(fqn, _openFlags, cos.PermRWR)
 }
 
 //
diff --git a/core/lom_xattr.go b/core/lom_xattr.go
index 49a149ae9e6..f2a2d2bf7c4 100644
--- a/core/lom_xattr.go
+++ b/core/lom_xattr.go
@@ -146,7 +146,7 @@ func (lom *LOM) lmfs(populate bool) (md *lmeta, err error) {
 	if !populate {
 		md = &lmeta{}
 	}
-	err = md.unmarshal(read)
+	err = md.unpack(read)
 	if err == nil {
 		_recomputeMdSize(size, mdSize)
 	} else {
@@ -165,7 +165,7 @@ func (lom *LOM) PersistMain() (err error) {
 		return
 	}
 	// write-immediate (default)
-	buf := lom.marshal()
+	buf := lom.pack()
 	if err = fs.SetXattr(lom.FQN, XattrLOM, buf); err != nil {
 		lom.Uncache()
 		T.FSHC(err, lom.FQN)
@@ -193,7 +193,7 @@ func (lom *LOM) Persist() (err error) {
 		return
 	}
 
-	buf := lom.marshal()
+	buf := lom.pack()
 	if err = fs.SetXattr(lom.FQN, XattrLOM, buf); err != nil {
 		lom.Uncache()
 		T.FSHC(err, lom.FQN)
@@ -211,7 +211,7 @@ func (lom *LOM) Persist() (err error) {
 }
 
 func (lom *LOM) persistMdOnCopies() (copyFQN string, err error) {
-	buf := lom.marshal()
+	buf := lom.pack()
 	// replicate across copies
 	for copyFQN = range lom.md.copies {
 		if copyFQN == lom.FQN {
@@ -237,7 +237,7 @@ func (lom *LOM) flushCold(md *lmeta, atime time.Time) {
 	if err := lom.syncMetaWithCopies(); err != nil {
 		return
 	}
-	buf := lom.marshal()
+	buf := lom.pack()
 	if err := fs.SetXattr(lom.FQN, XattrLOM, buf); err != nil {
 		T.FSHC(err, lom.FQN)
 	}
@@ -253,9 +253,9 @@ func (lom *LOM) flushAtime(atime time.Time) error {
 	return os.Chtimes(lom.FQN, atime, mtime)
 }
 
-func (lom *LOM) marshal() (buf []byte) {
+func (lom *LOM) pack() (buf []byte) {
 	lmsize := g.maxLmeta.Load()
-	buf = lom.md.marshal(lmsize)
+	buf = lom.md.pack(lmsize)
 	size := int64(len(buf))
 	debug.Assert(size <= xattrMaxSize)
 	_recomputeMdSize(size, lmsize)
@@ -292,7 +292,7 @@ func (md *lmeta) poprt(saved []uint64) {
 	md.Atime, md.atimefs, md.lid = int64(saved[0]), saved[1], lomBID(saved[2])
 }
 
-func (md *lmeta) unmarshal(buf []byte) error {
+func (md *lmeta) unpack(buf []byte) error {
 	const invalid = "invalid lmeta"
 	var (
 		payload                           string
@@ -404,7 +404,7 @@ func (md *lmeta) unmarshal(buf []byte) error {
 	return nil
 }
 
-func (md *lmeta) marshal(mdSize int64) (buf []byte) {
+func (md *lmeta) pack(mdSize int64) (buf []byte) {
 	var (
 		b8                    [cos.SizeofI64]byte
 		cksumType, cksumValue = md.Cksum.Get()
diff --git a/ec/bckencodexact.go b/ec/bckencodexact.go
index 7d19e805575..8e5e65120d0 100644
--- a/ec/bckencodexact.go
+++ b/ec/bckencodexact.go
@@ -123,7 +123,7 @@ func (r *XactBckEncode) afterECObj(lom *core.LOM, err error) {
 	if err == nil {
 		r.LomAdd(lom)
 	} else if err != errSkipped {
-		nlog.Errorf("Failed to erasure-code %s: %v", lom.Cname(), err)
+		nlog.Errorf("failed to erasure-code %s: %v", lom.Cname(), err)
 	}
 
 	r.wg.Done()
diff --git a/ec/ec.go b/ec/ec.go
index c7b9b0d74d4..99bce7d7574 100644
--- a/ec/ec.go
+++ b/ec/ec.go
@@ -207,7 +207,7 @@ func Init() {
 	xreg.RegBckXact(&encFactory{})
 
 	if err := initManager(); err != nil {
-		cos.ExitLogf("Failed to init manager: %v", err)
+		cos.ExitLog("Failed to init manager:", err)
 	}
 }
 
diff --git a/ec/getjogger.go b/ec/getjogger.go
index 4e3e34367af..4f26dc904fb 100644
--- a/ec/getjogger.go
+++ b/ec/getjogger.go
@@ -277,7 +277,7 @@ func (c *getJogger) restoreReplicatedFromDisk(ctx *restoreCtx) error {
 
 		w, err := ctx.lom.CreateWork(tmpFQN)
 		if err != nil {
-			nlog.Errorf("Failed to create file: %v", err)
+			nlog.Errorf("failed to create file: %v", err)
 			break
 		}
 		iReqBuf := newIntraReq(reqGet, ctx.meta, ctx.lom.Bck()).NewPack(g.smm)
@@ -285,10 +285,10 @@ func (c *getJogger) restoreReplicatedFromDisk(ctx *restoreCtx) error {
 		g.smm.Free(iReqBuf)
 
 		if err == nil && n != 0 {
-			// A valid replica is found - break and do close file handle
+			// found valid replica
 			err = cos.FlushClose(w)
 			if err != nil {
-				nlog.Errorf("Failed to flush and close: %v", err)
+				nlog.Errorf("failed to flush and close: %v", err)
 				break
 			}
 			ctx.lom.SetSize(n)
@@ -362,7 +362,7 @@ func (c *getJogger) requestSlices(ctx *restoreCtx) error {
 		if ctx.toDisk {
 			prefix := fmt.Sprintf("ec-restore-%d", v.SliceID)
 			fqn := fs.CSM.Gen(ctx.lom, fs.WorkfileType, prefix)
-			fh, err := ctx.lom.CreateWork(fqn)
+			fh, err := ctx.lom.CreateSlice(fqn)
 			if err != nil {
 				return err
 			}
@@ -420,7 +420,7 @@ func newSliceWriter(ctx *restoreCtx, writers []io.Writer, restored []*slice,
 	if ctx.toDisk {
 		prefix := fmt.Sprintf("ec-rebuild-%d", idx)
 		fqn := fs.CSM.Gen(ctx.lom, fs.WorkfileType, prefix)
-		file, err := ctx.lom.CreateWork(fqn)
+		file, err := ctx.lom.CreateSlice(fqn)
 		if err != nil {
 			return err
 		}
@@ -776,9 +776,9 @@ func (c *getJogger) restoreEncoded(ctx *restoreCtx) error {
 
 	// main replica is ready to download by a client.
 	if err := c.uploadRestoredSlices(ctx, restored); err != nil {
-		nlog.Errorf("Failed to upload restored slices of %s: %v", ctx.lom, err)
+		nlog.Errorf("failed to upload restored slices of %s: %v", ctx.lom, err)
 	} else if cmn.Rom.FastV(4, cos.SmoduleEC) {
-		nlog.Infof("Slices %s restored successfully", ctx.lom)
+		nlog.Infof("slices %s restored successfully", ctx.lom)
 	}
 
 	c.freeDownloaded(ctx)
diff --git a/ec/getxaction.go b/ec/getxaction.go
index 6bceef06b71..148cfb46f58 100644
--- a/ec/getxaction.go
+++ b/ec/getxaction.go
@@ -250,7 +250,7 @@ func (r *XactGet) decode(req *request, lom *core.LOM) {
 	req.tm = time.Now()
 
 	if err := r.dispatchRequest(req, lom); err != nil {
-		nlog.Errorf("Failed to restore %s: %v", lom, err)
+		nlog.Errorf("failed to restore %s: %v", lom, err)
 		freeReq(req)
 	}
 }
diff --git a/ec/putjogger.go b/ec/putjogger.go
index f9a435d26c1..996e5905713 100644
--- a/ec/putjogger.go
+++ b/ec/putjogger.go
@@ -448,7 +448,7 @@ func generateSlicesToDisk(ctx *encodeCtx) error {
 	cksumType := ctx.lom.CksumType()
 	for i := range ctx.paritySlices {
 		workFQN := fs.CSM.Gen(ctx.lom, fs.WorkfileType, fmt.Sprintf("ec-write-%d", i))
-		writer, err := ctx.lom.CreateWork(workFQN)
+		writer, err := ctx.lom.CreateSlice(workFQN)
 		if err != nil {
 			return err
 		}
@@ -495,7 +495,7 @@ func (c *putJogger) sendSlice(ctx *encodeCtx, data *slice, node *meta.Snode, idx
 			data.release()
 		}
 		if err != nil {
-			nlog.Errorln("Failed to send", hdr.Cname()+": ", err)
+			nlog.Errorln("failed to send", hdr.Cname()+": ", err)
 		}
 	}
 
diff --git a/ec/putxaction.go b/ec/putxaction.go
index de4715d6516..3210d9ffa67 100644
--- a/ec/putxaction.go
+++ b/ec/putxaction.go
@@ -215,7 +215,7 @@ func (r *XactPut) encode(req *request, lom *core.LOM) {
 	now := time.Now()
 	req.putTime, req.tm = now, now
 	if err := r.dispatchRequest(req, lom); err != nil {
-		nlog.Errorf("Failed to encode %s: %v", lom, err)
+		nlog.Errorf("failed to encode %s: %v", lom, err)
 		freeReq(req)
 	}
 }
@@ -226,7 +226,7 @@ func (r *XactPut) cleanup(req *request, lom *core.LOM) {
 	req.putTime, req.tm = now, now
 
 	if err := r.dispatchRequest(req, lom); err != nil {
-		nlog.Errorf("Failed to cleanup %s: %v", lom, err)
+		nlog.Errorf("failed to cleanup %s: %v", lom, err)
 		freeReq(req)
 	}
 }
diff --git a/ec/xaction.go b/ec/xaction.go
index 4a9ea563b18..f1218bd0fca 100644
--- a/ec/xaction.go
+++ b/ec/xaction.go
@@ -119,7 +119,7 @@ func newSliceResponse(md *Metadata, attrs *cmn.ObjAttrs, fqn string) (reader cos
 	attrs.Size = stat.Size()
 	reader, err = cos.NewFileHandle(fqn)
 	if err != nil {
-		nlog.Warningf("Failed to read file stats: %s", err)
+		nlog.Warningln("failed to read file stats:", err)
 		return nil, err
 	}
 	return reader, nil